fix(gitlab): discover workflow models in v2

[LeoParkerOu]

Issue for this PR

Closes #50421

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The v2 GitLab provider did not discover GitLab workflow models, so available workflow models were missing from the model picker.

This adds workflow model discovery during the GitLab catalog transform. It uses the configured GitLab credentials and instance URL, passes the current working directory to the GitLab provider, registers newly discovered models with their workflow references, and keeps existing catalog models unchanged.

How did you verify your code works?

• Prettier checks passed for the changed source and test files.
• tsgo --noEmit passed for the core package.
• Added regression coverage for discovered model metadata, workflow references, authentication headers, instance URL, and working directory.
• The focused Bun test currently stops during the repository's existing provider module initialization cycle with Cannot access 'GitLabPlugin' before initialization; the same failure occurs against the unmodified dev baseline.

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR that is not the current PR:

Related PR:

• PR feat(core): restore GitLab workflow discovery and add OAuth login #50422: feat(core): restore GitLab workflow discovery and add OAuth login
  • feat(core): restore GitLab workflow discovery and add OAuth login #50422

Why it's related: This PR also addresses GitLab workflow discovery (which is the issue closed by PR #50623). It may be working on the same or overlapping functionality. You should verify if these PRs are complementary or if one supersedes the other.