Add OAuth providers to test suite and improve test coverage

Tests updated:
- abort.test.ts: Add Google Gemini CLI, add retries
- agent.test.ts: Add OAuth providers (Anthropic, GitHub Copilot, Gemini CLI, Antigravity), add retries, remove timeouts
- context-overflow.test.ts: Handle Cerebras 429 status code
- image-tool-result.test.ts: Add OAuth providers
- overflow.ts: Detect 429 as overflow for Cerebras

Removed obsolete debug/one-off tests:
- copilot-initiator.test.ts
- gemini-3-flash-tool-calling.test.ts
- google-thought-signature.test.ts
- mistral-debug.test.ts
- mistral-empty-assistant.test.ts
- mistral-sdk.test.ts

packages/ai/test/context-overflow.test.ts

@@ -308,8 +308,8 @@ describe("Context overflow error handling", () => {
 			logResult(result);
 
 			expect(result.stopReason).toBe("error");
-			// Cerebras returns status code with no body
-			expect(result.errorMessage).toMatch(/4(00|13).*\(no body\)/i);
+			// Cerebras returns status code with no body (400, 413, or 429 for token rate limit)
+			expect(result.errorMessage).toMatch(/4(00|13|29).*\(no body\)/i);
 			expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
 		}, 120000);
 	});