feat(realtime): add SDK trace spans for realtime turns

[felmonon]

Summary

Related to #1845

Adds SDK-local tracing spans for realtime sessions:

• create agent spans for realtime turn lifecycles with current tool and handoff metadata
• wrap realtime function tool execution in SDK function spans
• wrap realtime handoff execution in SDK handoff spans
• respect tracing_disabled and avoid adding tool input/output payloads to tracing

This is intentionally scoped to SDK-emitted spans and does not claim to fix separate dashboard visibility issues.

Validation

• uv run pytest tests/realtime/test_session.py -q
• uv run pytest tests/realtime -q
• uv run ruff check src/agents/realtime/session.py tests/realtime/test_session.py
• uv run mypy src/agents/realtime/session.py tests/realtime/test_session.py
• uv run pyright src/agents/realtime/session.py tests/realtime/test_session.py
• git diff --check

[felmonon]

The formatting failure was fixed in 96541a4b.

The only remaining red check is tests (3.10), failing in make tests-asyncio-stability at tests/test_run_step_execution.py::test_multiple_tool_calls_surface_sleeping_post_invoke_failure_before_sibling_error.

This appears unrelated to this PR. The diff only touches:

• src/agents/realtime/session.py
• tests/realtime/test_session.py

I reran the same stability script locally under Python 3.10:

UV_PYTHON=3.10 bash .github/scripts/run-asyncio-teardown-stability.sh

It passed 5/5 locally.

I do not have permission to rerun the failed GitHub job.