Filter out unavailable issue labels from generated issues

[thomas-vilte]

Description

Please include a summary of the change and which issue is fixed. Also include relevant motivation and context.

I implemented a new function, filterToAvailableLabels, to ensure that only labels actually present in the repository are applied to generated issues. This function is integrated into the GenerateFromDiff, GenerateFromDescription, and GenerateFromPR methods of the IssueGeneratorService.

The motivation for this change is to prevent the application of non-existent labels. Issue templates or AI-generated content might suggest labels that have not been created in the GitHub repository, leading to inconsistencies or errors when attempting to apply them.

Fixes #

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• Unit tests
• Integration tests
• Manual test: (describe below)

I added a new unit test case in issue_generator_service_test.go to specifically verify that filterToAvailableLabels correctly drops labels that do not exist in the mock repository's available labels list.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Test Plan & Evidence

Suggested Manual Verification

• API/Backend: Attach JSON response or logs as evidence of correct behavior
• Performance: Ensure no significant latency increase
• Unit Tests: Run go test ./... and ensure all tests pass
• No Regressions: Verify that related features still work as expected