fix(prompt): omit tool_choice when prompt manages built-in tools

[OiPunk]

Summary

Fixes an issue where prompt-managed built-in tools can fail with:

Tool choice 'web_search_preview' not found in 'tools' parameter.

This happens when a prompt is supplied, local tools are empty, and a tool_choice value is still forwarded in the request.

What changed

• In OpenAIResponsesModel._fetch_response(...), when prompt mode is active and local tools are omitted, tool_choice is also omitted.
• Added regression test coverage to ensure tool_choice is omitted in this prompt-managed-tools path.

Why this is safe

• Only affects the prompt path where SDK-local tools are absent and prompt config is expected to manage tools.
• Existing behavior for non-prompt flows and prompt flows with explicit SDK tools remains unchanged.

Validation

• uv run --with ruff ruff check src/agents/models/openai_responses.py tests/test_openai_responses.py
• uv run --with mypy --with socksio mypy src/agents/models/openai_responses.py tests/test_openai_responses.py
• uv run --with pytest --with socksio pytest -q tests/test_openai_responses.py
• Targeted trace coverage run to confirm modified lines were exercised.

[chatgpt-codex-connector]

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