fix(openai): fix Responses API 400 errors for reasoning, tool calls, and ID handling (#11785)

* fix(openai): fix Responses API 400 errors for reasoning, tool calls, and ID handling

Three categories of OpenAI Responses API 400 errors are addressed:

1. "No tool call found for function call output with call_id" — function_call
   items are now always emitted when tool calls exist (even without fc_ IDs),
   and orphaned function_call_output items are removed in post-processing.

2. "Item provided without its required 'reasoning' item" — reasoning items
   without encrypted_content (e.g. from saved sessions) are removed, and the
   id is stripped from subsequent function_call items so the API doesn't
   reject them for referencing a missing reasoning item. Trailing reasoning
   items not followed by a valid successor are also removed.

3. "Expected an ID that begins with 'fc'" — responsesOutputItemIds (which
   accumulates both msg_ and fc_ IDs during streaming) is now filtered to
   only fc_-prefixed IDs for function_call items, preventing msg_ IDs from
   being used where fc_ IDs are required.

Co-authored-by: Luna <20552038+xxsLuna@users.noreply.github.com>
Co-authored-by: Robert Benko <10097827+Quazistax@users.noreply.github.com>
Co-authored-by: pallaprolus <162944051+pallaprolus@users.noreply.github.com>

* fix(openai-adapters): don't duplicate tool call args in Vercel stream converter

The tool-call event was re-emitting the full arguments that were already
streamed via tool-input-start/delta, causing JSON.parse failures in
consumers. Now tool-call only emits extra_content when a Gemini
thoughtSignature is present, and returns null otherwise.

---------

Co-authored-by: Luna <20552038+xxsLuna@users.noreply.github.com>
Co-authored-by: Robert Benko <10097827+Quazistax@users.noreply.github.com>
Co-authored-by: pallaprolus <162944051+pallaprolus@users.noreply.github.com>