feat(realtime): expose current_agent and context_wrapper on RealtimeSession

[Skyline-9]

Summary

Adds two read-only public properties to RealtimeSession: current_agent and context_wrapper.

RealtimeSession already exposes a public update_agent() setter and a public model property, but there is no public way to read which agent is currently active or to reach the run-context wrapper. Application code that runs outside the session's event loop (telemetry, per-agent metrics, routing decisions, background timers) currently has to read the private session._current_agent and session._context_wrapper, which couples user code to SDK internals.

This change makes those two reads first-class and additive:

• RealtimeSession.current_agent -> RealtimeAgent: the agent currently handling the session (the value set at construction and updated by update_agent() and by handoffs).
• RealtimeSession.context_wrapper -> RunContextWrapper[Any]: the run-context wrapper backing the session.

Both are read-only (no setters), typed consistently with existing internal usage, and documented as live, unsynchronized snapshots (a background reader can observe the value mid-handoff), matching the style of the existing model property.

Test plan

• Added tests/realtime/test_session.py::TestSessionAccessors covering: current_agent returns the initial agent, reflects the value after update_agent(...), and context_wrapper returns the same object the session was constructed with.
• Ran the standard verification stack from the repo root:
  • make format — pass
  • make lint — pass
  • make typecheck — pass (pyright + mypy)
  • uv run pytest -k realtime — pass (303 passed, 1 skipped)

Issue number

N/A (small additive ergonomics improvement). Happy to link a tracking issue if preferred.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass (ran make format, make lint, make typecheck, and the realtime test suite)
• If using Codex, I've run /review before submitting this PR

Compatibility notes

Purely additive. No new exported top-level symbols (properties on an existing class), no changes to constructor signatures or field order, and no behavior change to existing methods.