fix: support GitLab Duo workflows on self-managed instances

[MJVNOR]

Issue for this PR

Closes #50843

Type of change

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

What does this PR do?

Fixes GitLab Duo workflows on self-managed GitLab instances.

The GitLab provider now uses the configured instance URL when refreshing OAuth credentials and passes the current working directory to Duo workflow models, allowing gitlab-ai-provider to automatically detect the GitLab project context from the repository remote.

It also handles expired OAuth credentials gracefully. If the OAuth refresh fails, GitLab is disabled for the current session instead of preventing OpenCode from starting.

How did you verify your code works?

Tested locally on Windows with a self-managed GitLab instance.

• bun run --cwd packages/opencode typecheck passes.
• Built the Windows executable successfully; the build smoke test passed.
• Tested startup with stored GitLab OAuth credentials that were already expired.
• Confirmed GitLab returned 400 invalid_grant when OpenCode attempted to refresh the expired OAuth token.
• Confirmed OpenCode logged GitLab OAuth refresh failed; disabling GitLab provider for this session and continued initializing normally instead of failing startup.
• Verified workflow models receive the current working directory so gitlab-ai-provider can detect the GitLab project context from the repository remote.

Screenshots / recordings

N/A - no UI changes.

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, here are potentially related PRs:

1. fix(gitlab): discover workflow models in v2 - This addresses GitLab workflow model discovery, which is directly related to the workflow initialization changes in PR fix: support GitLab Duo workflows on self-managed instances #50844.

2. feat(core): restore GitLab workflow discovery and add OAuth login - This PR deals with GitLab workflow discovery and OAuth login functionality, which overlaps with the OAuth credential handling and workflow initialization in the current PR.

These PRs may share similar concerns around GitLab workflow setup and OAuth handling, though PR #50844 specifically focuses on self-managed instance support with expired credential handling.

[github-actions]

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