mcp: drain SSE messages when closing

[jstar0]

mcp: drain SSE messages when closing

SSEServerTransport.Close previously closed the session signal without draining buffered client messages. Because Read and ServeHTTP select between the message queue and the closed signal, a closed session could deliver stale messages or acknowledge a POST with 202 Accepted even though the message would never be processed.

This change drains buffered incoming messages when the server connection closes and checks the closed state around queue operations, so close races return io.EOF or 400 session closed. It adds regression tests for stale post-close reads and POST responses after close.

Tests:

• go test -race ./mcp -run '^(TestSSEServerConnCloseDrainsIncoming|TestSSEServerTransportServeHTTPAfterClose)$' -count=50 — passed.
• go test ./... — passed on the rebased head.
• go vet ./... — passed.
• gofmt -d mcp/sse.go mcp/sse_test.go — no changes reported.

Hosted checks for rebased head fd3a248297cd62389764587e2949a3cb988c6611 passed, including Test on Go 1.25 and 1.26, race-test, lint, client and server conformance, Analyze, and CodeQL.

Fixes #1191

[jstar0]

Rebased the single commit onto current main at 5bc078a. The fix remains limited to mcp/sse.go and mcp/sse_test.go. Focused race tests (50 runs), go test ./..., go vet ./..., and gofmt all pass; hosted checks for fd3a248 are green across Go 1.25/1.26, race, lint, client/server conformance, Analyze, and CodeQL. I updated the PR description to reflect these results. Would appreciate review when convenient.