refactor: add type-safe stream event interface to listenStream

[whitelonng]

Summary

Fixes type safety issue where listenStream used unsafe any for event payloads, providing no type checking for event structure or discriminated unions on eventType.

Changes

Type System

• Define comprehensive StreamEvent type hierarchy covering all event types:
  • Content events: StreamDeltaEvent, StreamTextEvent
  • Thinking events: StreamThinkingDeltaEvent, StreamThinkingSignatureEvent
  • Context events: StreamContextCompactedEvent (matches SessionContextCompaction schema)
  • Tool events: StreamToolStartEvent, StreamToolDeltaEvent, StreamToolDoneEvent, StreamToolEvent
  • Control events: StreamDoneEvent, StreamErrorEvent
• Update listenStream signature to use StreamEventWrapper with typed payload
• All events extend StreamEventBase with common runId, eventType, message fields

Example

Before (unsafe):

listenStream(runId, (event) => {
  if (event.payload.eventType === 'delat') { // typo, no compile error
    console.log(event.payload.text); // wrong field, no error
  }
});

After (type-safe):

listenStream(runId, (event) => {
  if (event.payload.eventType === 'delta') {
    console.log(event.payload.delta); // ✓ correct field, autocompleted
    console.log(event.payload.text);  // ✗ TypeScript error: Property 'text' does not exist
  }
});

Benefits

• ✅ Event structure validated at compile time
• ✅ IDE autocomplete shows available fields for each eventType
• ✅ Discriminated unions prevent accessing wrong fields
• ✅ Self-documenting: event structure centrally defined
• ✅ Catches eventType typos at compile time
• ✅ Type narrowing works correctly with switch/if statements

Test Plan

• ✅ 186 Rust unit tests pass
• ✅ TypeScript type check passes
• All existing listenStream usage sites now have proper type inference
• Event handlers correctly narrow types based on eventType checks

Breaking Changes

None for runtime behavior. Type-level only: code accessing non-existent event fields will now show TypeScript errors (which is the intended improvement).

Related

Complements PR #13 (appInvoke type safety) by adding type safety to the stream event system.