[OmarAlJarrah]

Summary

Test files were hard to navigate. compilers/openapi held its four source files among 53 test files, most of them end-to-end tests that compile a whole spec. Across the repo, many test files had names that sort next to no source file, and conformance_test.go ran to 4,659 lines. This PR keeps unit tests beside the code they test, gives the OpenAPI compiler's end-to-end tests a package of their own, names test files after the file they test, and writes the placement rule down. No non-test Go file changes.

• compilers/openapi/e2e, a package of test files only, now holds the end-to-end tests: the conformance corpus, the lowering-specification checks, goldens, fuzz targets with their seed corpus, benchmarks, and the whole-compiler scenarios. Files move whole, so git log --follow and blame track them. The root keeps only the tests of its own four files. Two exported defaults (DefaultStreamingMediaTypes, DefaultExtensionPromotions) had been covered only by end-to-end tests, so each gets a root unit test. sourcetable_test.go gets fixtures of its own instead of borrowing overlay_test.go's.
• Shared helpers in e2e move to its helpers_test.go; until now each lived in whichever file first needed it.
• conformance_test.go keeps the runner, the case table and the corpus reader. Each case's assertion moves to a conformance_<family>_test.go file for its construct (types, composition, params, content, and so on).
• TestXML_NodeTypeIsReadAtEveryVersion moves from operation's tests, whose code it never reaches, to e2e's later_version_test.go, as test: an XML nodeType test lowers no operation yet sits in operation's #1074 proposed.
• Tests that drove one internal lowering through a root-only fixture move to the package they test, onto the fixture that package already had: the constraint and component-schema tests go to internal/schema, the service test to internal/operation, the unpreservable tests to e2e. Five root tests are deleted rather than moved, because a test in the package under test already pins what they did:
  • TestIntern_IdempotentOnSamePointer and TestPrimID_SecondCallReuses: compile's TestTypes_InternIsIdempotentAndRecordsBeforeBuilding and TestTypes_PrimRefInternsOnceAndNamesNoSource kill all three mutants planted in Intern/PrimRef (a rebuild on revisit, a fresh ID per call, a re-minted ID on a second reach). The root tests kill only two of them.
  • The three TestSiteAt_* tests: annotation's TestAt_* kill each mutant of At they kill (an empty $ref read as a reference, a reference site with no node, every site a declaration).
• Test files are named after the file they test when every test in them names a declaration of one source file, as in load/entry_internal_test.go → load_entry_internal_test.go. 47 files are renamed and two are split: compile_test.go into types_test.go and diags_test.go, and harness's internal_test.go by the file each test covers. Files named for a scenario several source files decide, and whole-package checks such as ir/nofloat_test.go, keep their names.
• archtest now checks package-qualified test citations (pkg.TestName) in Markdown. Its pattern used to start at Test, so a citation naming the package a test had moved out of passed unread.
• CONTRIBUTING gains a "Where a test goes" section. A test goes where the code that decides what it asserts lives; e2e takes only what the compiler does as a whole, so an internal package's test that compiles a whole spec to reach its own code stays in that package, where its coverage counts. Each rule names the check that fails when it is missed, or says that nothing does. The conformance step, README commands, docs/compilers/openapi.md citations, the root guidance's layout and docs/architecture.md's package tree all point at the new homes. Comments the moves would have made false are corrected, commands in Go comments included; where a symbol or test name can stand in for a file name, they now use it. Two comments in moved files cited tests deleted before this change (TestConformance_UnwitnessedIRFields, TestWriteCompiled_ReadOnlyDirFails) and now name what holds those facts today.

Deliberately left alone:

• No integration or performance package. engine's tests are the integration layer's own and count toward its coverage. A benchmark of unexported code has to sit in its package, so benchmarks stay beside what they measure, and only the compiler's whole-document benchmark moves with e2e.
• Scenario test files whose tests run through a package's entry point and assert what several files decide keep their names (load's stream, nesting, plus_refs, documentbudget; resolve's beside; merge's conflict; the CLI's command, completion and help tests).
• auth_test.go keeps its TestAuth_ tests, as decided when auth.go was split.
• Nesting test files under their source file in an editor's tree view is editor configuration and is not committed.

Test plan

• Test inventory against main: go test -list '.*' ./... lists 3,481 tests on main and 3,478 here. The difference is the two root unit tests added, the five dominated tests deleted, and TestFillPropertyExamples_RefSiblingExampleBindsTheSite renamed to TestFillPropertyAnnotations_…, since the function it tests is fillPropertyAnnotations now.
• Full suite under go test -json -race ./...: 12,274 passing tests and subtests on main (fuzz seeds included) against 12,271 here, differing in the same names only. The four TestLoad_AFileURIReadsTheFileItNames subtests are named after a random temp directory, so they differ in name only.
• go run ./cmd/morphic-covergate over that run's profile: all 15,144 statements covered, the same total across the same 52 packages as main's profile, so no package's statements left the count.
• make fmt build vet lint lint-planted bench-smoke exit 0, with golangci-lint at the pinned version. make fuzz FUZZTIME=2s finds and runs the three fuzz targets in their new package.
• Declaration dumps (every top-level declaration with its comment) are byte-identical before and after for the helper gathering, the conformance split and the two file splits. Of the 47 renames, 45 change nothing else (git diff -M --name-status reports R100); the root's SourceTable tests also carry the fixtures they gained, and main_edge_cases_test.go a corrected test citation. The files moved into e2e keep at least 76% similarity, the rest being the helpers that moved out of them and, in later_version_test.go, the nodeType test that moved in.
• Mutants planted in the code under test, run against the moved tests in their new homes:
  • schema's constraint stamping and its 3.0 dialect flag, killed;
  • the dedup that docs/micro-compiler-design.md §4.1 says TestConstraints_HoistedSubSchemaBadBoundSingleError holds, killed;
  • an allowEmptyValue entry kept under the wrong reason, killed by TestPreserve_AllReasonsReachable;
  • the root's two new default tests, each killed by a changed default;
  • a nodeType never read, killed by the nodeType test in its new home.
• archtest: a citation naming a moved test by its old package goes red, and the same citation with the right package passes. TestLoweringSpec_TestsAreDeclared still goes red on a stale citation.
• go test ./compilers/openapi/e2e -run 'TestGolden|TestConformance' -update leaves testdata/ unchanged, and a deleted conformance golden comes back byte-identical.
• Every go test command quoted in a Markdown document or a Go comment lists at least one test with its -run filter, and every test name a Go comment cites is declared.
• Each commit's tree vets and passes the tests of the packages it touches on its own.
• A trial merge with the one open PR, chore: add a review agent built from past PR fixes #888, is clean. On the merged tree archtest reports seven stale test names in chore: add a review agent built from past PR fixes #888's own review records, the same seven it reports on main merged with chore: add a review agent built from past PR fixes #888.

Closes #1074