fix(labeler): assert assessment-URL origin, not prefix (clears CodeQL)

CodeQL js/incomplete-url-substring-sanitization (high) flagged a test
assertion doing `assessmentUrl.startsWith("https://labels.example")` —
a prefix that `https://labels.example.evil.com` also satisfies. The
production URL builder (assessmentUrl, notification-triggers.ts) is sound
(`new URL(path, base)`); this was a test-only false positive. Parse the
URL and assert origin equality instead — stronger, and clears the rule.

Author: ascorbic

apps/labeler/test/prolonged-error.test.ts

@@ -487,7 +487,7 @@ describe("sweep parity and finalization invariant", () => {
 
 		expect(notice).not.toBeNull();
 		expect(notice?.subject).toContain("couldn't complete");
-		expect(notice?.assessmentUrl.startsWith(SERVICE)).toBe(true);
+		expect(new URL(notice!.assessmentUrl).origin).toBe(SERVICE);
 	});
 
 	it("notifyAssessmentOutcome never notifies on an error assessment (finalization stays silent)", async () => {