feat: normalize stream_event contract + add live integration coverage

[cpacker]

Summary

• add explicit SDKStreamEventPayload union to model both content_block-style and message_type-style stream chunks
• keep strict typing for known message chunk events while allowing unknown/raw passthrough events
• add extractStreamTextDelta helper for consumers to parse assistant/reasoning text across both stream shapes
• add unit tests for stream-event parsing
• add opt-in live integration test suite + fixture recording docs/scripts

Verification

• bun run check
• bun test src/tests/stream-events.test.ts