feat(realtime): add generation_tick event support

[AdirAmsalem]

Summary

• Add generation_tick event support to surface periodic billing updates (total billed seconds) during authenticated realtime sessions
• generation_ended is parsed but intentionally not exposed — unreliable on client disconnect, overlaps with connection_change, and insufficient_credits is already covered by the error event
• Mirrors the implementation already shipped in the TypeScript SDK (feat(realtime): add generationTick event for billing updates sdk#81)

Usage

def on_tick(data):
    print(f"Billed so far: {data.seconds}s")

client.on("generation_tick", on_tick)

Changes

• messages.py — GenerationTickMessage and GenerationEndedMessage Pydantic models, added to IncomingMessage discriminated union
• webrtc_connection.py — on_generation_tick callback, handles both events in _handle_message()
• webrtc_manager.py — on_generation_tick added to WebRTCConfiguration, wired through _create_connection()
• client.py — Callback list, emit/dispatch, buffer flush, on()/off() support for "generation_tick"
• init.py — Export GenerationTickMessage for user type hints