sloproxy

feat(executor): add session isolation for `grok-composer` models master

- Introduced `xaiRequiresIsolatedConversation` to enforce session ID generation for `grok-composer` models. - Updated request preparation logic to handle isolated conversations by setting `prompt_cache_key` and `x-grok-conv-id`. - Added unit tests with coverage for session isolation, stateless models, and explicit `prompt_cache_key` scenarios. Closes: #3750


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