[Platform][OpenAi] Route commentary phase to ThinkingDelta in streaming responses

[Amoifr]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #1979
License	MIT

The OpenAI Responses streaming API emits both commentary and final_answer as response.output_text.delta events. The current str_contains($type, 'output_text') guard in Gpt\ResultConverter::convertStream() treats them identically, so the visible assistant text gets duplicated whenever the model emits a commentary that mirrors the final answer (the bug reported by @wimwinterberg).

Phases are now tracked per item_id via response.output_item.added, and response.output_text.delta is routed accordingly:

• commentary → ThinkingStart / ThinkingDelta, finalized by ThinkingComplete on response.output_item.done
• final_answer / unknown phase / missing item_id → TextDelta (backward compatible)

The response.reasoning_summary_text.* path now uses its own buffer ($currentReasoningSummary) to avoid colliding with the commentary buffer.

The fix follows the approach @wimwinterberg sketched in the issue thread; this PR formalizes it and adds an exhaustive test suite (9 tests) covering:

• commentary routed to thinking deltas, never to TextDelta
• final_answer still emits TextDelta as before
• delta with no item_id keeps its TextDelta (backward compat)
• multi-delta accumulation into a single ThinkingComplete
• empty commentary item emits nothing
• reasoning_summary regression guard
• the real-world case (commentary + final_answer carrying the same content)
• truly interleaved streams (both items open, deltas alternating by item_id)
• aborted stream (response.output_item.done never arrives)

Thanks @OskarStark for the nudge on the issue.