feat: implement checkpoint service integration with git operations (#317427)

* feat: implement checkpoint service integration with git operations

- Added new methods to IAgentHostGitService for capturing working trees, committing trees, updating refs, deleting refs, resolving refs, and computing file diffs between refs.
- Integrated checkpoint service into AgentService and related classes to manage session data and git state.
- Enhanced AgentSideEffects to capture turn checkpoints and baseline checkpoints for sessions.
- Updated SessionDataService to emit events before deleting session data, allowing for async cleanup.
- Modified session database schema to include checkpoint references and added methods to manage them.
- Updated tests to cover new checkpoint functionalities and ensure proper integration with existing services.

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* feat: enhance agent host services with checkpoint integration and update diff parsing

* test: update turn complete test to await promise resolution for accurate assertions

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>