fix(mcp): map tool-not-found to CodeNotFound via registry sentinel

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

internal/mcp/server_test.go

@@ -117,3 +117,31 @@ func TestPostBodyTooLarge(t *testing.T) {
 	assert.NotEqual(t, http.StatusOK, rr.Code, "oversized body must not return 200")
 	assert.Equal(t, http.StatusBadRequest, rr.Code)
 }
+
+func TestToolsCallToolNotFound(t *testing.T) {
+	srv := newServer(t)
+	// Initialize to get a session ID.
+	init := postJSON(t, srv, map[string]any{
+		"jsonrpc": "2.0",
+		"id":      1,
+		"method":  "initialize",
+		"params":  map[string]any{"protocolVersion": "2025-06-18"},
+	}, "")
+	sid := init.Header().Get("Mcp-Session-Id")
+
+	rr := postJSON(t, srv, map[string]any{
+		"jsonrpc": "2.0",
+		"id":      2,
+		"method":  "tools/call",
+		"params":  map[string]any{"name": "nonexistent", "arguments": map[string]any{}},
+	}, sid)
+	require.Equal(t, http.StatusOK, rr.Code)
+
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+	rpcErr, ok := resp["error"].(map[string]any)
+	require.True(t, ok, "expected error field in response")
+	code := int(rpcErr["code"].(float64))
+	assert.Equal(t, -32002, code, "expected CodeNotFound (-32002) for missing tool")
+	assert.NotEmpty(t, rpcErr["message"])
+}