fix: include auth headers in responses websocket handshake

[maxpetrusenkoagent]

Summary

Fixes the Responses WebSocket handshake header merge so it includes the OpenAI client's auth_headers. In recent openai SDK versions, Authorization lives in AsyncOpenAI.auth_headers, not default_headers, so the Agents SDK WebSocket path could connect without auth and receive HTTP 401.

This keeps the existing precedence shape narrow:

• client auth headers are merged first
• client default headers can override them, case-insensitively
• per-request extra_headers still apply last and can replace or omit inherited headers

Test plan

• uv run pytest tests/models/test_openai_responses.py::test_websocket_model_prepare_websocket_request_includes_client_auth_headers tests/models/test_openai_responses.py::test_websocket_model_prepare_websocket_request_preserves_client_header_overrides tests/models/test_openai_responses.py::test_websocket_model_prepare_websocket_request_omit_removes_inherited_header tests/models/test_openai_responses.py::test_websocket_model_prepare_websocket_request_replaces_header_case_insensitively tests/models/test_openai_responses.py::test_websocket_model_prepare_websocket_request_skips_not_given_header_values -q
• uv run pytest tests/models/test_openai_responses.py tests/test_config.py -q
• make lint
• uv run ruff format --check src/agents/models/openai_responses.py tests/models/test_openai_responses.py
• uv run mypy src/agents/models/openai_responses.py tests/models/test_openai_responses.py --exclude site
• uv run python -m pyright --project pyrightconfig.json

Issue number

Closes #3133

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[maxpetrusenkoagent]

Verification update from Hermes Agent:

• Focused websocket header tests: passed (5 passed)
• Focused model/config tests: passed (136 passed)
• make lint: passed
• make format: passed, no files changed
• Changed-file mypy: passed
• Full pyright via uv run python -m pyright --project pyrightconfig.json: passed
• GitHub PR checks: none reported for this fork branch at time of verification
• Read-only Codex autoreview against origin/main...HEAD: CLEAN, no blockers

Note: a full make tests run was attempted locally before opening the PR and hit one Dapr memory-session collection error plus one async cancellation timing failure. The timing test passed when rerun directly; the focused tests covering this change passed after the final patch.