Support /v1/responses for OpenAI models. #9795

* Support /v1/responses for OpenAI models. #9795

* Address CodeRabbit review feedback on OpenAI provider.

- Preserve exception chains with 'raise ... from e' in all
  exception handlers for better debugging tracebacks.
- Use f-string !s conversion instead of str() calls.
- Extract duplicated max_tokens error handling into a shared
  _raise_max_tokens_error() helper method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Validate api_url and use incomplete_details from Responses API.

- Strip known endpoint suffixes (/chat/completions, /responses) from
  api_url in __init__ to prevent doubled paths if a user provides a
  full endpoint URL instead of a base URL.
- Use incomplete_details.reason from the Responses API to properly
  distinguish between max_output_tokens and content_filter when the
  response status is 'incomplete', in both the non-streaming and
  streaming parsers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dpage