Enable fixed comment on issues and refactor GitHub operations

[xueyulinn]

Summary

This PR enables the backend service to post a fixed comment to a GitHub pull request issue thread and refactors the GitHub integration code into a clearer layered structure.

Related Issue

N/A

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Test
• CI/CD
• Chore

Changes Made

• Added GitHub integration modules under codehawk/github/:
  • auth.py for GitHub App installation token creation
  • client.py for GitHub comment posting
  • constants.py for GitHub API constants
  • dto.py for pull request webhook DTOs
  • webhook.py for webhook payload parsing
• Implemented the flow to post a fixed comment to a PR issue thread.
• Updated pull_request_summary_service to orchestrate:
  • webhook parsing
  • installation token creation
  • GitHub client construction
  • issue comment posting
• Updated github_handler to call the refactored PR summary flow.
• Updated settings and JWT utility usage to support the GitHub App flow cleanly.

How to Test

pytest tests -q
ruff check .
mypy codehawk

Then run the backend and trigger a supported pull_request webhook event to verify that the service posts the fixed comment successfully.

Checklist

• Code builds successfully
• Tests added or updated
• Existing tests pass
• Documentation updated if needed
• No secrets or sensitive data included
• Breaking changes documented

Breaking Changes

None.

Additional Notes

Validation completed locally:

• pytest tests -q -> 7 passed
• ruff check . -> passed
• mypy codehawk -> passed

There is still a non-blocking pytest cache permission warning in the local environment, but it does not affect test results.