Validate some untrusted GitHub API responses using validator system

[Copilot]

GitHub API Response Validation - Complete ✅

Supports makeGitHubAPIRequest function returning unknown (untrusted data) instead of typed responses. Callers validate responses using the validator system before use.

Room for improvement here, either adding more methods to be validated or setting certain fields to required (would need confirmation from github on which fields are actually required)

[joshspicer]

@copilot For vJobInfo you're validating too strictly!

2025-11-13 18:07:16.529 [error] [GitHubService] Failed to validate job info response: Required field 'pull_request' is missing
2025-11-13 18:07:16.531 [error] Failed to provide new chat session item: Failed delegating to cloud agent. Please try again later.
Inner Error: Invalid job info response: Required field 'pull_request' is missing

[Copilot]

@copilot For vJobInfo you're validating too strictly!

2025-11-13 18:07:16.529 [error] [GitHubService] Failed to validate job info response: Required field 'pull_request' is missing
...

Fixed in f403ecb. Made pull_request optional in both the JobInfo interface and vJobInfo validator since the API can return job info without this field. Added a test case to verify jobs without pull_request validate successfully.