test(mcp): pin session concurrency, document error codes, assert id round-trip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mcp/jsonrpc_test.go

@@ -15,6 +15,9 @@ func TestRequestUnmarshal(t *testing.T) {
 	require.NoError(t, json.Unmarshal(raw, &req))
 	assert.Equal(t, "2.0", req.JSONRPC)
 	assert.Equal(t, "initialize", req.Method)
+	// ID is opaque; encoding/json decodes JSON numbers into float64 by default.
+	// We don't type-assert in the server — we echo it back unchanged.
+	assert.Equal(t, float64(1), req.ID)
 }
 
 func TestErrorResponseShape(t *testing.T) {