feat(realtime): add generationTick event for billing updates

[AdirAmsalem]

Summary

• Surfaces the generation_tick WebSocket message as a public generationTick event on the realtime client, letting consumers track billed seconds in real-time during authenticated sessions
• Parses generation_ended internally but intentionally does not expose it — devs should use connectionChange for disconnect and error for insufficient credits

Usage

client.on("generationTick", ({ seconds }) => {
  console.log(`Billed so far: ${seconds}s`);
  updateBillingUI(seconds);
});

Changes

File	Change
types.ts	Add GenerationTickMessage, GenerationEndedMessage types + IncomingWebRTCMessage union
webrtc-connection.ts	Parse both events in handleSignalingMessage, emit generationTick via mitt
client.ts	Add generationTick to Events type, wire listener to buffered event emitter

Design Decisions

• generation_ended not exposed: It's unreliable (won't arrive on client-initiated disconnect or network drop) and overlaps with connectionChange: "disconnected". The insufficient_credits case is already covered by the error event.
• camelCase naming (generationTick): Matches existing SDK conventions (connectionChange, promptAck, sessionId). Wire format stays snake_case.
• No breaking changes: Adds to the existing Events type — on/off just gains a new event key.

---

Note

Low Risk
Additive event/type wiring only; no changes to core connection/retry logic or outbound protocol behavior.

Overview
Surfaces server-side generation_tick updates as a new public realtime client event generationTick, allowing consumers to track billed generation seconds during a session.

Adds new websocket message types GenerationTickMessage and GenerationEndedMessage to the signaling type union; the connection layer emits generationTick via its internal emitter and explicitly parses-but-does-not-expose generation_ended.

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