OAI Responses API json schema support, Cohere2 MoE template parser json schema support, improvements to responses streaming compatibility

[boondocklabs]

Overview

Improves OpenAI Responses API compatibility and adds constrained schema support in Responses API, and the Cohere2 MoE chat template parser.

Additional information

• Convert Responses API text.format settings into the corresponding Chat Completions response_format
• Enable JSON-schema-constrained output for Cohere2 MoE
• Improve Responses API streaming event compatibility:
  • Adds monotonically increasing sequence_number values.
  • Adds output_index and content_index where required.
  • Emits complete response snapshots for creation and completion events.
  • Emits the reasoning content-part lifecycle, including reasoning_text.done, content_part.done, and output_item.done.
  • Ensures reasoning is finalized before text or tool-call output begins.
  • Avoids emitting duplicate reasoning completion events during finalization.
  • Includes consistent response metadata and usage information in the completed event.

Testing

• Added coverage for:
  • Conversion of Responses API text.format.type = "json_schema" into response_format.
  • Preservation of the schema name, strict mode, and schema body.
  • Cohere2 MoE parsing of a schema-constrained JSON response with reasoning and tools enabled.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes, ChatGPT used for code exploration and analyzing logic, all code edits and validation performed manually.

[pwilkin]

This looks fine, but please split this into two separate PRs. The Cohere chat parser fixes and the OAI response fixes are completely unrelated.

[boondocklabs]

This looks fine, but please split this into two separate PRs. The Cohere chat parser fixes and the OAI response fixes are completely unrelated.

Converted this PR to a draft and submitted #26018 for the Cohere parser. I can only open a single PR at a time, so I'll submit the other when it's merged.