mcp: drain SSE messages when closing

[jstar0]

Fixes #1191.

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.
• Checks the closed state before and after queue operations so close races return io.EOF or 400 session closed.
• Adds regression tests for stale post-close reads and POST responses after close.

Tests:

• go test ./mcp -run '^TestSSE(ServerConnCloseDrainsIncoming|ServerTransportServeHTTPAfterClose|Server|Handler_DiscoverFallsBackToInitialize|ClientTransport_HTTPErrors|405AllowHeader)$' -count=20
• go test -race ./mcp -run '^TestSSE(ServerConnCloseDrainsIncoming|ServerTransportServeHTTPAfterClose|Server)$' -count=20
• go vet ./...
• go test ./... (the MCP package still has the pre-existing localhost protection failure where the 0.0.0.0 via localhost rejects evil.com case receives HTTP 200 instead of 403; all other packages pass)