fix(realtime): forward client default headers to websocket connections

[sylvesterkaczmarek]

Summary

• forward user-configured default_headers into Realtime WebSocket handshakes
• preserve authentication header precedence, while per-call extra_headers remain the final override
• apply the behavior to both sync/async Realtime clients and the legacy beta.realtime path
• add regression coverage for proxy-auth headers and override precedence

Fixes #1975

Test plan

• uv run ruff format src/openai/resources/realtime/realtime.py src/openai/resources/beta/realtime/realtime.py tests/lib/test_websocket_redirects.py
• uv run ruff check src/openai/resources/realtime/realtime.py src/openai/resources/beta/realtime/realtime.py tests/lib/test_websocket_redirects.py
• uv run pytest -o addopts= -q tests/lib/test_websocket_redirects.py -k default_headers
• git diff --check

All focused verification steps passed before the branch was reduced to its single final commit.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 45d6f6ff83

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Merge authentication headers case-insensitively

When a caller supplies a differently cased authentication key such as default_headers={"authorization": "Bearer stale"}, this case-sensitive dictionary expansion retains both that value and the generated Authorization header. WebSocket header names are case-insensitive, so the handshake receives duplicate credentials rather than the intended generated-auth precedence and may be rejected; the same pattern occurs in the sync and beta paths. Normalize header names or remove case-insensitive collisions before connecting, and cover this case in both sync and async authentication tests.

AGENTS.md reference: AGENTS.md:L41-L45

Useful? React with 👍 / 👎.