fix(client): preserve Streamable HTTP request provenance

[edenbuilds]

Summary

• preserve the originating client request ID across a Streamable HTTP POST SSE response stream
• expose it through the existing MessageExtraInfo callback carrier for server-initiated JSON-RPC requests
• keep standalone GET messages and normal responses backward-compatible
• add a client patch changeset and transport regression test

Verification

• Regression proof: before the source change, the focused test received no attributed server request.
• After the fix: streamableHttp.test.ts passes 70/70.
• @modelcontextprotocol/client typecheck passes.
• @modelcontextprotocol/core-internal typecheck passes.
• Prettier and git diff --check pass.

Fixes #2659

[changeset-bot]

🦋 Changeset detected

Latest commit: d363e64

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: d363e64

[edenbuilds]

Refreshed this branch onto the current upstream/main without rewriting history (d363e64). The diff against upstream remains limited to the Streamable HTTP provenance fix, its regression test, and the changeset. All 14 reported CI checks are green; ready for maintainer review.