Deferred tool calls now preserve namespace field

Patch Changes

• c65c952: fix(openai): round-trip namespace on function_call input items

  When tool_search dispatches a deferred tool, the resulting function_call carries a namespace field identifying which deferred-tool group the model picked. #14789 preserved this on the read side (providerMetadata.openai.namespace), but the write side still serialized function_call input items without namespace. Multi-step / multi-turn conversations then failed with Missing namespace for function_call '<name>'. ... Round-trip the model's function_call item with its namespace field included.

  convert-to-openai-responses-input.ts now reads namespace from providerOptions.openai.namespace (or providerMetadata.openai.namespace) on tool-call parts and includes it on the serialized function_call item, mirroring how itemId is round-tripped.