fix(mcp): add GET endpoint for SSE stream connections

[octo-patch]

Summary

Some MCP clients (e.g. Gemini CLI) use GET /mcp-server/http to establish SSE streams for server-initiated messages, per the MCP Streamable HTTP transport specification. Without a registered GET route, these requests returned 404 Not Found.

This PR adds a GET /mcp-server/http endpoint that delegates to StreamableHTTPServerTransport.handleRequest(), following the same stateless pattern used by the existing POST handler (creates a new transport+server per request with sessionIdGenerator: undefined for isolation).

Observed before fix:

• POST /mcp-server/http → works correctly
• GET /mcp-server/http → 404 Not Found (breaks Gemini CLI SSE handshake)

After fix:

• Both GET and POST requests are handled correctly

Related Linear tickets, Github issues, and Community forum posts

Fixes #28137

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with Backport to Beta, Backport to Stable, or Backport to v1 (if the PR is an urgent fix that needs to be backported)

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