Add a pathPrefix setting for services mounted under a prefix

[kubukoz]

The generated client builds its URL from each operation's @http URI verbatim. That is correct when the model describes the whole path, but a service is often mounted under a prefix the model doesn't mention — a server framework that derives one from a trait, or a reverse proxy — and then every generated request misses it and 404s.

This adds a pathPrefix setting that is prepended to each operation's URI.

{
  "plugins": {
    "ts-codegen": {
      "outFile": "generated.ts",
      "pathPrefix": "/internal/v1"
    }
  }
}

// before
const url = `/things/${encodeURIComponent(String(input.id))}`
// after
const url = `/internal/v1/things/${encodeURIComponent(String(input.id))}`

Notes on the design

It applies to the Storybook mocks too. The mock router matches path segments one at a time, so the prefix arrives there as literal segments ({ literal: 'internal' }, { literal: 'v1' }, …) rather than as one concatenated string. Without that the mocks would silently stop matching the very client they stand in for.

A plain string, not trait-derived. Which prefix a deployment sits behind isn't a property of the model, and a trait-aware version would have to bake in one framework's conventions. A caller that does have a convention (a trait, a service version field) can compute the string and pass it.

Normalization accepts what people actually write. A leading slash is optional, a trailing one is ignored, and "/" or whitespace means no prefix — so "internal/v1", "/internal/v1" and "/internal/v1/" are equivalent. Without it "/internal/v1/" would emit v1//things. There's also an edge case where a URI of just / combined with a prefix must not leave a trailing slash.

Default is "", which is a no-op: the committed typecheck/src/generated.ts sample is byte-identical.

Surfaces

All four the existing excludeServices setting covers: the smithy-build plugin, the CLI, the sbt plugin (tsCodegenPathPrefix), and the README.

Two wiring details worth a look in review:

• The CLI takes its args positionally, so a prefix is passed as a 4th arg after excludeServices; the sbt plugin sends an empty string for excludes when only a prefix is set. The existing parse already ignored a 4th arg, so nothing changes for current callers.
• tsCodegenPathPrefix is folded into the sbt plugin's cache key — otherwise changing only the prefix wouldn't re-run codegen.

Release note: base version moved to 0.5

generate gains a parameter. The default value keeps it source-compatible, but a defaulted parameter still changes the JVM signature, so the two-arg overload disappears — and MiMa caught it against the published 0.4.0:

* static method generate(Model,Set)String in class TsCodegenPlugin
  does not have a correspondent in current version

So the second commit bumps tlBaseVersion to 0.5. I went with the bump rather than a ProblemFilters.exclude: generate is the documented programmatic entry point, so the break is genuinely user-visible, unlike the TsWriter filter that was dropped when 0.4 opened (a class no caller could reach). Happy to switch to a filter if you would rather keep the next release a patch, but I do not think this one qualifies.

Worth flagging that this needs a v0.5.0 tag when it ships — sbt-typelevel checks the release tag against tlBaseVersion.

Verification

• 47 core tests pass (38 pre-existing + 9 new, covering both consumers and the normalization edge cases).
• The scripted sbt test asserts the prefixed URL and the prefixed mock segments, which exercises the real sbt-plugin → forked-CLI → SPI path including the positional-arg handoff.
• scalafmtCheckAll, scalafmtSbtCheck, headerCheckAll, doc all green, and mimaReportBinaryIssues green with the 0.5 base version (mimaPreviousArtifacts is empty against an unpublished baseline, which is the point of the bump).
• Sample regenerated and confirmed unchanged via git status rather than tsCodegenSampleCheck — see tsCodegenSampleCheck can never fail: it regenerates the file it is checking #22 for why that check can't be trusted for this.

[kubukoz]

Superseded by #25. A single static setting can't express this: one codegen run can emit several services that need not share a prefix. #25 replaces it with a transformPath extension point that sees each service and operation.