feat: Make the DSL generator runnable by plugin authors

[TimothyJones]

Stacked on #1386 (claude/plugin-loading) — completes the DSL generator todo: it can now be run outside the ContractCase repository, via a new CLI command.

The generator package

@contract-case/definition-generator's entry point was a maintainer script with hardcoded /Users/home/office/... paths that ran generation as an import side effect. Now:

• The entry point is a proper API barrel
• generatePluginDsl() loads plugin packages by name — with the same loading rules as loadPlugins for a test run, via the loadPluginModuleContents helper extracted into case-plugin-base (this PR's first commit moves it there from BoundaryPluginLoader, which now consumes it; no behaviour change)
• Core DSL generation moved to a bin script with repo-relative paths, run with npm run generate:core-dsls, which properly awaits generation (previously errors were unhandled rejections and the two language passes raced)
• A plugin without a dsl property is reported as a CaseConfigurationError (BAD_DSL_DECLARATION) instead of a plain Error, so the CLI classifies it as a configuration problem rather than a crash
• Fixed a latent Java-generator bug: interaction parameters without a jsonPropertyName defaulted to _case:interaction:<name> where the TypeScript generator produces _case:mock:<name>. No shipped declaration relied on the default, so no generated output changes.

The CLI command

ContractCase generate-plugin-dsl @yourorg/your-plugin --languages java,ts --output-dir .

Files are written to the conventional paths beneath the output directory (src/main/java/... for Java, src/boundaries/dsl/... for TypeScript).

Verification

• 5 new tests generate from a fixture plugin package loaded by name and assert the emitted files land in the right places for both languages; existing generator suite (39 tests) passes, snapshots unchanged
• The CLI command was smoke-tested end to end, including the error classification for a DSL-less plugin
• npm run generate:core-dsls reproduces the committed Java DSL — the only diffs (committed separately as a chore) are a documentation link that had been updated in the declaration data since the files were last generated, and two prettier-java line-wrapping changes from the current formatter version

Known limitation (documented, not addressed)

Generated Java classes for third-party plugins currently land under ContractCase's own package namespace (io.contract_testing.contractcase.dsl.<kind>.<category>), with the plugin's category as the leaf. The docs note this and invite an issue if it's a problem — making the namespace configurable touches the renderer and the marker-interface imports, so it felt like its own piece of work.

Docs updated: the generate-plugin-dsl command in the CLI reference, the plugin DSL page's work-in-progress caveats replaced with usage instructions, regeneration instructions in docs/maintainers/DslGenerators.md, and the todo ticked.

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https://claude.ai/code/session_014mD6p9EEmftbpBQHZZBWB1

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