Filter unexecutable local calls and add a deterministic fallback at the iteration limit

[shibayan]

Summary

On the final permitted iteration, the function-invocation loop removes local function declarations. If the provider nevertheless emits a local call or approval, TypeScript returns content it will neither execute nor remove, despite the public contract promising a final answer with no unanswered local tool calls.

Python hardened this behavior with per-update filtering and the deterministic fallback text:

Function invocation limit reached before a final answer could be produced.

Required implementation

• On the final request, remove local function declarations and send toolChoice: "none".
• Treat only actionable local function_call content and local function_approval_request content as unexecutable. Use the existing hosted-approval predicate; hosted approvals are provider-owned and survive.
• In streaming mode, filter each update before accumulation/yield. Do not add a full-final-round gate: retained updates continue flowing as they arrive.
• Preserve text, reasoning, metadata, raw representation, hosted/informational content, and provider-executed informationalOnly call/result pairs.
• If removing local content leaves a metadata-bearing update, retain it. Omit only a truly empty update.
• Append the exact fallback only when finalization leaves no non-blank user-visible answer and no hosted approval. Do not replace retained content to make room for it.

Acceptance criteria

• The final request has no local function declarations and uses toolChoice: "none".
• A provider-emitted actionable local call or local approval on the final round is absent from yielded updates and the folded response and is never executed.
• A streaming test proves retained content is yielded before the source stream completes; no new full-round buffering is introduced.
• Mixed updates retain text/reasoning/metadata while removing only unexecutable local content.
• A metadata-only update remains visible after its local call is removed.
• Provider-executed informationalOnly call/result pairs and hosted approvals survive unchanged in awaited and streamed modes.
• When no visible answer or hosted approval remains, the exact deterministic fallback is appended in both modes.
• When visible content or a hosted approval remains, no fallback is added.
• Awaited output equals the fold of streamed output for the same provider sequence.