[v1.x] fix(server): parse Accept media types exactly

[SnowSky1]

Backport of #2481 to v1.x. Related to #2480.

Motivation and Context

The v1 Streamable HTTP server still validates Accept with raw substring checks. Values such as application/jsonx and text/event-stream-bogus therefore satisfy the required concrete media types, and q=0 exclusions are treated as support.

This backport parses the comma-separated media ranges, compares exact media-type essences case-insensitively, handles quoted commas, and requires a positive RFC 9110 quality value.

How Has This Been Tested?

• npx vitest run test/shared/mediaType.test.ts — 12 passing
• npx vitest run test/server/streamableHttp.test.ts — 197 passing
• npm run typecheck — clean
• npm run lint — clean
• ESM and CJS TypeScript builds — clean
• full non-E2E suite — 52 files and 1659 assertions passed locally; native Windows emitted two pre-existing post-test stdio JSON parse errors
• GitHub CI — all applicable checks green: build, Node 18/24 unit and E2E suites, client/server conformance, and package preview; the publish job is expectedly skipped for a PR

Breaking Changes

None. This tightens invalid Accept negotiation only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally (all assertions pass; see the Windows stdio note above)
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The companion main implementation is #2481. This follows the same two-branch delivery pattern used for the related Content-Type fixes#2441 and #2444.

[changeset-bot]

🦋 Changeset detected

Latest commit: a1cc606

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@modelcontextprotocol/sdk@2738

commit: a1cc606

[Parker-Fawcett]

Good backport - exact media type parsing is the right call. One thing to watch: the Accept header contract between client and server should be documented somewhere, or different implementations will keep diverging on edge cases. This is the class of problem rebuild-dossier solves mechanically. Paper: https://arxiv.org/abs/2608.23616