Bump gitlab-ai-provider to 6.7.0 for GPT-5.5 model support

[vglafirov]

The GitLab Duo provider needs to be updated to support the new GPT-5.5 model (gpt-5.5-2026-04-23) which is now available in the ai-gateway model selection config.

The current version (6.6.0) does not include duo-chat-gpt-5-5 in its MODEL_MAPPINGS. Upstream models.yml exposes the gpt_5_5 entry with proxy_provider: openai and a 1.05M token context, so the model is already routable through the GitLab AI Gateway — opencode just needs the provider bump to surface it.

Follow-up to closed #18850 (which did the same bump for the GPT-5.4 family).

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