fix(ai): handle same-provider different-model handoff in OpenAI Responses API

When switching between OpenAI models (e.g., gpt-5-mini to gpt-5.2-codex),
function_call IDs with fc_ prefix trigger pairing validation errors because
OpenAI tracks which fc_xxx IDs were paired with rs_xxx reasoning items.

The fix omits the id field for function_calls from different models, which
avoids the pairing validation while keeping call_id for matching with
function_call_output.

Fixes #886

packages/ai/CHANGELOG.md

@@ -7,6 +7,10 @@
 - Added `headers` option to `StreamOptions` for custom HTTP headers in API requests. Supported by all providers except Amazon Bedrock (which uses AWS SDK auth). Headers are merged with provider defaults and `model.headers`, with `options.headers` taking precedence.
 - Added `originator` option to `loginOpenAICodex()` for custom OAuth client identification
 
+### Fixed
+
+- Fixed OpenAI Responses API 400 error "function_call without required reasoning item" when switching between models (same provider, different model). The fix omits the `id` field for function_calls from different models to avoid triggering OpenAI's reasoning/function_call pairing validation ([#886](https://github.com/badlogic/pi-mono/issues/886))
+
 ## [0.49.2] - 2026-01-19
 
 ### Added