feat: Added gitlab as vcsprovider in codereference

[SahilJat]

Thanks for submitting a PR! Please check the boxes below:

• [ x ] I have read the Contributing Guide.
• [ x ] I have added information to docs/ if required so people know about the feature.
• [ x ] I have filled in the "Changes" section below.
• [ x ] I have filled in the "How did you test this code" section below.

Changes

Closes #7134

Please describe.
Types: Added GITLAB to VCSProvider and included vcs_provider in CodeReferencesRepositoryCount.

Models: Added vcs_provider to FeatureFlagCodeReferencesScan with a default of "github". Generated the corresponding migration.

Services: Updated _get_permalink() to handle GitLab's specific file permalink format (/-/blob/{revision}/{file_path}#L{line_number}).

Serializers: Exposed vcs_provider in the upload serializer and the count serializer.

Views: Updated the Feature list API (code_references_counts) to expose the vcs_provider.

How did you test this code?

Please describe.
Added unit tests for _get_permalink() covering both public and self-hosted GitLab URLs (with and without trailing slashes).

Added view tests for the POST .../code-references/ endpoint to verify it accepts "gitlab", rejects invalid providers, and defaults to "github".

Added view tests for the Detail view to verify GitLab scans return the correct /-/blob/ permalinks.

Updated feature list API tests to assert vcs_provider is present in code_references_counts.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[vercel]

@SahilJat is attempting to deploy a commit to the Flagsmith Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.28%. Comparing base (9e3edac) to head (90133a2).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7201      +/-   ##
==========================================
- Coverage   98.35%   98.28%   -0.08%     
==========================================
  Files        1348     1349       +1     
  Lines       50691    50603      -88     
==========================================
- Hits        49859    49736     -123     
- Misses        832      867      +35     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.