Pin LiteLLM to patched 1.84.3 release

[SunnySoldier357]

Summary

• pin the Python SDK and LiteLLM proxy image to 1.84.3, covering the published LiteLLM security fixes and follow-up route hardening
• allow LiteLLM's background logging worker to claim callbacks before short-lived event loops close
• add strict regression coverage for direct completions, rollout processing, MCP policy calls, Tau2, and streaming

Test plan

• Exact-lock Python 3.12 compatibility suite: 44 passed, 1 skipped
• Strict multi-event-loop suite (-W error): 14 passed
• Live Fireworks SDK completions, streaming and non-streaming
• LiteLLM proxy image startup, health check, and live streaming/non-streaming completions
• uv lock --check, pip check, and Ruff

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