Enforce OpenAI-compatible `tool_choice` contracts for Gateway HTTP Chat Completions and Responses client function tools.
- Add shared request normalization and post-run enforcement for required and pinned client function tool choices.
- Buffer streaming output until the tool-choice contract is satisfied, so failed runs do not leak partial assistant prose.
- Document the client-function-tool scope and add regression coverage for Chat/Responses success and failure cases.
Thanks @Lellansin for the contribution.
Proof: exact-head CI passed for `79fa0947360d307cf4ecffe713489cdf5db61093` in run `26714604449`; focused gateway tests passed locally.