feat(agent_session): allow update_options(stt=, tts=) for runtime STT/TTS swap

[SamarthUrs18]

Motivation

There is currently no public API to swap STT/TTS at runtime. The workaround
requires reaching into private internals:

session._activity._audio_recognition.update_stt(new_stt_node)

update_stt() already exists and is already the right mechanism — this PR
just exposes it through the existing update_options() surface.

Use case

Mid-call language switching. User says "speak in Kannada" → agent swaps the
Sarvam STT language code and the TTS voice without dropping the call.

Verified end-to-end with livekit-plugins-sarvam against a local LiveKit
server in console mode: the Sarvam STT WebSocket closes and reconnects
with the new language code on update_options(stt=...); TTS takes effect
on the next synthesis call.

A reference agent that uses this as an LLM-callable tool lives in
examples/dev/sarvam_language_swap.py (gitignored under
examples/dev/*). It exposes a switch_language(language_code) tool and
swaps STT/TTS based on the user's request.

Changes

• AgentSession.update_options gains stt= and tts= kwargs.
• AgentActivity.update_options forwards them and calls
  audio_recognition.update_stt(...) for STT; TTS takes effect on the next
  synthesis call because tts_node reads activity.tts per call.
• When the agent was constructed with its own STT/TTS instance, the swap is
  mirrored onto agent._stt / agent._tts so activity.stt /
  activity.tts continue to prefer the agent-bound instance (matching the
  existing resolution order).

The stt/tts parameter names shadow the same-named module imports inside
the method body. Since the body never references the modules after the
parameter list, no aliasing is required; this is documented in a Note
block in the docstring.

Tests

Five new tests in tests/test_agent_session.py:

• test_update_options_stt_swaps_session_and_rewires_activity — verifies
  session._stt is swapped, activity.stt resolves to the new STT, and
  audio_recognition.update_stt is called with the agent's bound
  stt_node.
• test_update_options_tts_swaps_session_and_agent_resolves_to_new_tts —
  TTS swap mirrors the session-level behavior.
• test_update_options_stt_mirrors_to_agent_when_agent_has_own_stt —
  confirms the agent-level mirror when the agent has its own STT.
• test_update_options_stt_none_disables_pipeline — stt=None clears
  the pipeline.
• test_update_options_stt_unchanged_when_not_provided — passing only
  endpointing_opts leaves STT/TTS untouched.

All 53 tests in test_agent_session.py pass; make check is clean.

Backwards compatibility

update_options was already sync (not async), so the new kwargs are
purely additive. All existing callers continue to work unchanged. The
parameter list keeps *-only style and uses NotGivenOr[T | None] to
match the rest of the codebase's "explicit None to disable" convention.

[CLAassistant]

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