fix(sessions): ignore in-memory partial events

[pragnyanramtha]

Summary

• return early from InMemorySessionService.appendEvent for partial streaming events
• keep partial events from mutating the caller session, persisted in-memory session, or lastUpdateTime
• add regression coverage for ignored partial events

Validation

• npm install
• npm run build --workspace core
• npx vitest run --project unit:core --dir core/test/sessions in_memory_session_service_test.ts
• npx prettier --check core/src/sessions/in_memory_session_service.ts core/test/sessions/in_memory_session_service_test.ts
• npx eslint core/src/sessions/in_memory_session_service.ts core/test/sessions/in_memory_session_service_test.ts
• git diff --check

Notes

• npx vitest run --project unit:core core/test/sessions/in_memory_session_service_test.ts failed before building because global setup imports @google/adk from missing dist; building core first resolved it.
• npm install reports existing audit findings in dependencies: 9 low, 4 moderate, 13 high.