Test(handler): fix failing TestSearchHandlerUpdateRejectsInvalidSearchID assertion (#17622)

### Summary

`TestSearchHandlerUpdateRejectsInvalidSearchID` fails on `main`:

```
--- FAIL: TestSearchHandlerUpdateRejectsInvalidSearchID (0.00s)
    search_handler_test.go:98: expected 'no authorization' in message, got No authorization.
```

`UpdateSearch` answers an unauthorized `search_id` with `"No
authorization."` (`internal/handler/search.go:337`), which mirrors the
Python API's message verbatim, but the assertion searched for the
lowercase `"no authorization"`.

The fix lowercases the response before the substring check rather than
changing the handler's message: the capitalized text is the contract the
Python API exposes, so the test — not the handler — was wrong.

`bash build.sh --test -run TestSearchHandler ./internal/handler/`
passes.

internal/handler/search_handler_test.go

@@ -94,7 +94,7 @@ func TestSearchHandlerUpdateRejectsInvalidSearchID(t *testing.T) {
 	if resp["code"] != float64(common.CodeAuthenticationError) {
 		t.Fatalf("expected code 109, got %v", resp["code"])
 	}
-	if !strings.Contains(resp["message"].(string), "no authorization") {
+	if !strings.Contains(strings.ToLower(resp["message"].(string)), "no authorization") {
 		t.Fatalf("expected 'no authorization' in message, got %v", resp["message"])
 	}
 }