Implement Database-Backed Session Storage (DatabaseSessionService) for ADK Java #665

ADK Java currently provides only in-memory session storage, which is not suitable for production environments that require durable, consistent, and queryable session/state/event storage.

Projects building on ADK often need:

• Persistent session storage across restarts
• Reliable event logging and retrieval
• Concurrency-safe updates
• Support for standard relational databases
• Schema versioning for continuous delivery

Describe the solution you'd like

Introduce a production-ready database-backed session service (DatabaseSessionService) that provides durable storage for sessions, events, and state.

Key characteristics:

• Pure JDBC implementation with zero ORM dependencies
• HikariCP connection pooling for high performance
• Flyway migrations for schema versioning and zero-downtime deployments
• Multi-database support: PostgreSQL, MySQL, H2, Cloud Spanner (SQLite not supported)
• 3-tier state storage: Separate tables for app-level, user-level, and session-level state
• Pessimistic locking for safe concurrent updates
• Dialect-aware JSON handling (JSONB for PostgreSQL, JSON for MySQL, CLOB for others)
• Reactive API with RxJava 3 (Single/Maybe/Completable)
• Event filtering and pagination
• Comprehensive test coverage using H2 and integration tests on actual databases (PostgreSQL, MySQL, Spanner) running in Docker

This service offers a robust, scalable alternative to the current in-memory session implementation without introducing heavy ORM dependencies.

Describe alternatives you've considered

Providing this as an external extension/library

• Persistent sessions are commonly required; shipping this in-core provides a better out-of-the-box experience.

Continuing with in-memory storage only

• Not viable for production deployments requiring durability or concurrency guarantees.

Using Hibernate/JPA

• Adds significant dependency overhead and complexity; pure JDBC provides better control and performance.

Additional context

The Python ADK includes a production-grade DatabaseSessionService that is widely used in real-world workloads (including GKE deployments). Adding equivalent persistent session storage to ADK Java would
provide consistency across ADK environments and reduce the need for custom user-built solutions.

An implementation is available in PR: #700