Pass expanded_ai_logging FF to AI Gateway

This is a high priority MR for Switch to Chat Agent V2 (gitlab-org#13533 - closed). Please prioritize the review and merge.

What does this MR do and why?

We currently have no visibility on the process-level logging hence it's very hard to investigate an issue on production. This MR expands the logging only when the expanded_ai_logging feature flag is enabled. We'll mainly use this flag for the Switch to Chat Agent V2 (&13533 - closed).

For the FF definition, see https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/feature_flags/development/expanded_ai_logging.yml.

AI Gateway counter-part: feat(chat): support expanded_ai_logging feature... (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1352 - merged)

Related to Extend feature flag to log LLM input/output in ... (#485490 - closed)

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