chore: update CI configuration and enhance test stability

- Added deterministic API key and environment variables in e2e-tests.yml to ensure consistent test behavior.
- Refactored CodexProvider tests to improve type safety and mock handling, ensuring reliable test execution.
- Updated provider-factory tests to mock installation detection for CodexProvider, enhancing test isolation.
- Adjusted Playwright configuration to conditionally use external backend, improving flexibility in test environments.
- Enhanced kill-test-servers script to handle external server scenarios, ensuring proper cleanup of test processes.

These changes improve the reliability and maintainability of the testing framework, leading to a more stable development experience.

apps/ui/src/lib/http-api-client.ts

@@ -349,6 +349,7 @@ export const verifySession = async (): Promise<boolean> => {
     const response = await fetch(`${getServerUrl()}/api/settings/status`, {
       headers,
       credentials: 'include',
+      signal: AbortSignal.timeout(5000),
     });
 
     // Check for authentication errors
@@ -390,6 +391,7 @@ export const checkSandboxEnvironment = async (): Promise<{
   try {
     const response = await fetch(`${getServerUrl()}/api/health/environment`, {
       method: 'GET',
+      signal: AbortSignal.timeout(5000),
     });
 
     if (!response.ok) {