fix(client): fire onerror only once when the standalone GET stream fails

[pablopupo]

Ports #880 to v2, following the maintainer request in #868. Fixes #868.

Motivation and Context

_startOrAuthSse reported a failure through onerror and then rethrew it. The standalone GET and reconnection call sites caught that same rejection and reported it again, so one stream failure could produce two callbacks.

The helper now only propagates failures. Each call site that owns a stream reports the failure once. resumeStream() keeps its existing contract: the returned promise rejects and onerror is called once. The 401/403 authentication retry path continues to return through the recursive call without taking error ownership.

How Has This Been Tested?

Four event-driven regressions cover:

• a failed standalone GET;
• close() aborting an in-flight standalone GET;
• resumeStream() rejection plus one onerror callback;
• a failed resumption GET started by _send.

The focused transport tests pass with 73 tests. The full client suite passes with 801 tests, and the repository's non-e2e package matrix, check:all, build:all, documentation build, distribution type smoke test, and changeset check also pass.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[changeset-bot]

🦋 Changeset detected

Latest commit: c4ad5e3

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

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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commit: c4ad5e3