fix(mcp): drop strict session-id requirement on POST /mcp

The claude.ai connector's MCP transport proxy does not reliably
propagate the Mcp-Session-Id header issued during initialize. With the
previous strict gate (return 400 plain text "missing or invalid
Mcp-Session-Id"), every tools/list and tools/call from claude.ai
failed and the Anthropic proxy surfaced it as:

  Streamable HTTP error: {"jsonrpc":"2.0","id":N,"error":
    {"code":-32600,"message":"Anthropic Proxy: Invalid content from server"}}

— because the plain-text 400 response is not valid JSON-RPC.

All tools the gitea-mcp server exposes are stateless single-shot
calls, so there is no functional reason to gate them on a session.
brain-mcp and supervisor-mcp don't gate either, and claude.ai works
against them fine. Match that behavior: keep issuing Mcp-Session-Id
on initialize for clients that want to use it, but stop rejecting
calls that don't send one back.

Test renamed PostWithoutSessionRejected → PostWithoutSessionAccepted
and updated to assert the tools/list response shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/mcp/server.go

@@ -56,7 +56,6 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// initialize is the only method allowed without a session.
 	if req.Method == "initialize" {
 		sid := s.opts.Sessions.Issue()
 		w.Header().Set("Mcp-Session-Id", sid)
@@ -68,11 +67,12 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	sid := r.Header.Get("Mcp-Session-Id")
-	if !s.opts.Sessions.Valid(sid) {
-		http.Error(w, "missing or invalid Mcp-Session-Id", http.StatusBadRequest)
-		return
-	}
+	// Mcp-Session-Id is advisory: we issue one on initialize and accept it back,
+	// but every tool the gitea-mcp server exposes is stateless single-shot, so
+	// we do not gate non-initialize calls on it. The claude.ai connector's
+	// transport proxy is observed to not propagate the session header reliably,
+	// and the spec allows servers to be sessionless. Compare with brain-mcp /
+	// supervisor-mcp, which never required a session at all.
 
 	switch req.Method {
 	case "tools/list":