chore: bump gitlab-ai-provider to 6.15.0

[vglafirov]

Issue for this PR

Closes #47791

Type of change

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

What does this PR do?

Bumps the pinned gitlab-ai-provider dependency from 6.14.0 to 6.15.0 in packages/core and packages/opencode (plus the resulting bun.lock update).

6.15.0 adds a mapping for GitLab Duo's newly published GPT-6 Astra model: duo-chat-gpt-6-astra -> OpenAI gpt-6-astra, routed through the Responses API. Without the bump the model is absent from the provider's MODEL_MAPPINGS, so it never appears in the model picker even though GitLab's API already serves it.

No opencode-side code was needed — GitLab model resolution is entirely driven by the provider package's mapping table combined with the models.dev metadata entry (anomalyco/models.dev#6499). This is a version bump only.

How did you verify your code works?

After bun install, confirmed the workspace resolves the published package (not a leftover file: link) and that it contains the new model:

$ grep '"version"' packages/core/node_modules/gitlab-ai-provider/package.json
  "version": "6.15.0",
$ grep -c "gpt-6-astra" packages/core/node_modules/gitlab-ai-provider/dist/index.js
1

The model is listed by the CLI:

$ bun run --cwd packages/opencode src/index.ts models | grep gpt-6-astra
gitlab/duo-chat-gpt-6-astra

And a live round-trip against the GitLab Duo endpoint returns a real completion:

$ bun run --cwd packages/opencode src/index.ts run "Reply with exactly: astra-ok" \
    --model gitlab/duo-chat-gpt-6-astra
> plan · duo-chat-gpt-6-astra
astra-ok

(Run with OPENCODE_MODELS_PATH pointed at a locally built models.dev JSON containing the pending duo-chat-gpt-6-astra entry, since that PR is not merged yet.)

Screenshots / recordings

Not a UI change.

Checklist

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

[opencode-agent]

Dependency and supply-chain review passed. The 6.15.0 tarball integrity matches bun.lock; it was published through npm trusted publishing from GitLab CI, has no install lifecycle hooks, and keeps the same runtime dependency set. Compared with 6.14.0, the shipped code adds only the GPT-6 Astra mapping plus generated source-map/version metadata. OSV and deps.dev report no known advisory for gitlab-ai-provider@6.15.0, and Bun’s full-lock audit output is byte-identical before and after this PR, so the update introduces no known vulnerability delta.