Implement logic orchestration for Git Pull/Push operations

[wanling0000]

Closes #12350

Final PR based on draft: JabRef#714

This PR implements the logic-layer orchestration for Git push and pull operations with semantic merging support. It enables JabRef to semantically merge .bib files when possible, avoiding manual conflict resolution for simple changes.

This is part of ongoing work to support full Git-based collaboration for .bib files. Further tests and UI integration are planned.

Steps to test

This PR implements the first part of Git sync support by enabling automatic semantic merges when there are no conflicts. The scenario is tested via TDD in GitSyncServiceTest, simulating the following steps:

• Alice creates an initial commit and pushes it to the remote.
• Bob clones the repo and modifies entry b, then pushes.
• Alice modifies entry a, then pulls.
  Since they changed different entries, we expect a clean merge without user intervention. The orchestrator is GitSyncService, and helper utilities and value objects are temporarily located in org.jabref.logic.git.util.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[subhramit]

IntelliJ probably replaced this

Suggested change

	* @return true if closing the tab was isSuccessful
	* @return true if closing the tab was successful