Merge https://github.com/google/adk-python/pull/3324
**Problem:**
ADK seems to not pass output/input transcription `finished` flag from the Gemini.
**Solution:**
Relaxation of checking conditions of valid llm_response for input/output transcription.
### Testing Plan
Unit test `test_receive_transcript_finished` checks if input/output transcription message with no text but `finished` flag is received.
**Unit Tests:**
- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.
<img width="785" height="373" alt="image" src="https://github.com/user-attachments/assets/6d870e9f-1372-4808-91a9-38578c1b3729" />
**Manual End-to-End (E2E) Tests:**
Configure Gemini Agent to produce input & output transcriptions. Observe incoming transcript messages - to see if the finished flag appears at the end agents & users statement.
### Checklist
- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.
### Additional context
The mentioned `finished` flag can be obtained in native Gemini APIs like via Websocket but not via ADK.
Co-authored-by: Hangfei Lin <hangfei@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3324 from ChrisQlasty:fix/transcript_finish e7b8e5e5f5fac1d2dfd495f2dbadb19ed4328b0c
PiperOrigin-RevId: 828987996
Krzysztof CzuszynskiandCopybara-Service