fix: clear session ID on HTTP 404 per MCP spec §Session Management

[Christian-Sidak]

Summary

• Clears _sessionId in _send() (POST path) when the server responds with HTTP 404 and a session ID was set
• Clears _sessionId in _startOrAuthSse() (GET path) with the same condition
• Adds 3 regression tests: POST path clears session ID, GET path clears session ID, non-404 errors do NOT clear session ID

Background

The MCP spec (2025-03-26) §Session Management point 4 states:

When a client receives HTTP 404 in response to a request containing an Mcp-Session-Id, it MUST start a new session by sending a new InitializeRequest without a session ID attached.

Previously, both code paths threw StreamableHTTPError without clearing this._sessionId, leaving the transport stuck with a stale session ID on all subsequent calls. Every follow-up send() would keep attaching the dead Mcp-Session-Id header and receiving 404 indefinitely.

Test plan

• should clear session ID on 404 POST (per spec §Session Management) — POST path clears _sessionId after 404
• should clear session ID on 404 GET SSE (per spec §Session Management) — GET path clears _sessionId after 404
• should NOT clear session ID on non-404 errors — 500 errors leave _sessionId intact
• All 56 existing client-transport tests still pass

Fixes #1708

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Latest commit: 6a46396

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/client	Patch

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