Merge https://github.com/google/adk-python/pull/3926
### Link to Issue or Description of Change
**1. Link to an existing issue (if applicable):**
- Related: #3916
**2. Or, if no issue exists, describe the change:**
**Problem:**
While `DatabaseSessionService` already supports PostgreSQL through SQLAlchemy, there is no documentation or sample code showing users how to configure and use it.
**Solution:**
Add a comprehensive sample under `contributing/samples/postgres_session_service/` that demonstrates:
- How to configure `DatabaseSessionService` with PostgreSQL
- The auto-generated database schema (sessions, events, app_states, user_states tables)
- Connection URL format and configuration options
- A working sample agent with session persistence
### Testing Plan
**Unit Tests:**
- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.
This is a documentation-only change (new sample), so no new unit tests are required. Existing tests continue to pass.
**Manual End-to-End (E2E) Tests:**
Tested locally with the following steps:
1. Started PostgreSQL using `docker compose up -d`
2. Set environment variables:
```bash
export POSTGRES_URL=postgresql+asyncpg://postgres:postgres@localhost:5432/adk_sessions
export GOOGLE_CLOUD_PROJECT=$(gcloud config get-value project)
export GOOGLE_CLOUD_LOCATION=us-central1
export GOOGLE_GENAI_USE_VERTEXAI=true
```
3. Ran `pip install google-adk asyncpg greenlet` to install the required packages
4. Ran `python main.py` - session created successfully
5. Ran `python main.py` again - previous session resumed with event history
6. Verified tables and rows created in PostgreSQL (sessions, events, app_states, user_states)
### Checklist
- [x] I have read the 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
This PR adds documentation and a working sample for an already-supported feature. The DatabaseSessionService class already handles PostgreSQL through its DynamicJSON type decorator which uses JSONB for PostgreSQL.
Files added:
- contributing/samples/postgres_session_service/README.md - Comprehensive guide
- contributing/samples/postgres_session_service/agent.py - Sample agent
- contributing/samples/postgres_session_service/main.py - Usage example
- contributing/samples/postgres_session_service/compose.yml - Local PostgreSQL setup
- contributing/samples/postgres_session_service/\_\_init\_\_.py - Package init
Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3926 from hiroakis:feat-support-pg-for-conversation a5279d4fb2a63699f384c670928d77d882c25a25
PiperOrigin-RevId: 868816317
Hiroaki Sano
657acfadbb