feat: Add DatabaseSessionService with Hibernate and Flyway support

Implements a production-ready database-backed session service that provides
persistent storage for ADK sessions, events, and state using JPA/Hibernate.

Key features:
- Hibernate 6.6 + HikariCP connection pooling for optimal performance
- Flyway migrations for schema versioning and zero-downtime deployments
- Support for PostgreSQL, MySQL, H2, and other RDBMS
- Thread-safe operations with pessimistic locking for concurrent updates
- Comprehensive test coverage with H2 in-memory database
- Dialect-aware JSON storage (JSONB for PostgreSQL, CLOB for others)
- Configurable event filtering and pagination

Database dependencies are non-optional in core module for ease of use,
consistent with Python ADK's approach where DatabaseSessionService is
in the main package.

Author: prasadskarmarkar

core/pom.xml

@@ -104,6 +104,17 @@
       <groupId>com.fasterxml.jackson.dataformat</groupId>
       <artifactId>jackson-dataformat-yaml</artifactId>
     </dependency>
+    <!-- Jackson JAXB annotations module for ObjectMapper.findAndRegisterModules() -->
+    <dependency>
+      <groupId>com.fasterxml.jackson.module</groupId>
+      <artifactId>jackson-module-jaxb-annotations</artifactId>
+    </dependency>
+    <!-- JAXB API needed by jackson-module-jaxb-annotations on modern JDKs -->
+    <dependency>
+      <groupId>jakarta.xml.bind</groupId>
+      <artifactId>jakarta.xml.bind-api</artifactId>
+      <version>4.0.0</version>
+    </dependency>
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
@@ -189,6 +200,70 @@
       <artifactId>opentelemetry-sdk-testing</artifactId>
       <scope>test</scope>
     </dependency>
+
+    <!-- Database Session Service Dependencies -->
+    
+    <!-- Hibernate ORM - JPA implementation for object-relational mapping -->
+    <dependency>
+      <groupId>org.hibernate.orm</groupId>
+      <artifactId>hibernate-core</artifactId>
+    </dependency>
+    
+    <!-- Jakarta Persistence API - Standard JPA annotations and interfaces -->
+    <dependency>
+      <groupId>jakarta.persistence</groupId>
+      <artifactId>jakarta.persistence-api</artifactId>
+    </dependency>
+    
+    <!-- HikariCP - High-performance JDBC connection pool -->
+    <dependency>
+      <groupId>com.zaxxer</groupId>
+      <artifactId>HikariCP</artifactId>
+    </dependency>
+    
+    <!-- Hibernate HikariCP Integration - Enables HikariCP as Hibernate's connection provider -->
+    <dependency>
+      <groupId>org.hibernate.orm</groupId>
+      <artifactId>hibernate-hikaricp</artifactId>
+    </dependency>
+    
+    <!-- Database Drivers -->
+    
+    <!-- H2 Database - In-memory database for testing -->
+    <dependency>
+      <groupId>com.h2database</groupId>
+      <artifactId>h2</artifactId>
+      <scope>test</scope>
+    </dependency>
+    
+    <!-- PostgreSQL Driver - JDBC driver for PostgreSQL databases (optional for production use) -->
+    <dependency>
+      <groupId>org.postgresql</groupId>
+      <artifactId>postgresql</artifactId>
+      <optional>true</optional>
+    </dependency>
+    
+    <!-- MySQL Driver - JDBC driver for MySQL databases (optional for production use) -->
+    <dependency>
+      <groupId>com.mysql</groupId>
+      <artifactId>mysql-connector-j</artifactId>
+      <optional>true</optional>
+    </dependency>
+    
+    <!-- Database Migration -->
+    
+    <!-- Flyway Core - Database schema migration and versioning tool -->
+    <dependency>
+      <groupId>org.flywaydb</groupId>
+      <artifactId>flyway-core</artifactId>
+    </dependency>
+    
+    <!-- Flyway PostgreSQL Support - PostgreSQL-specific migration support -->
+    <dependency>
+      <groupId>org.flywaydb</groupId>
+      <artifactId>flyway-database-postgresql</artifactId>
+      <scope>runtime</scope>
+    </dependency>
   </dependencies>
   <build>
     <resources>

core/src/main/java/com/google/adk/runner/Runner.java

@@ -56,9 +56,12 @@
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
 import javax.annotation.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /** The main class for the GenAI Agents runner. */
 public class Runner {
+  private static final Logger logger = LoggerFactory.getLogger(Runner.class);
   private final BaseAgent agent;
   private final String appName;
   private final BaseArtifactService artifactService;
@@ -542,7 +545,16 @@ public Flowable<Event> runLive(
               invocationContext
                   .agent()
                   .runLive(invocationContext)
-                  .doOnNext(event -> this.sessionService.appendEvent(session, event))
+                  .flatMap(
+                      event ->
+                          this.sessionService
+                              .appendEvent(session, event)
+                              .toFlowable()
+                              .onErrorResumeNext(
+                                  error -> {
+                                    logger.warn("Failed to append event to session", error);
+                                    return Flowable.just(event);
+                                  }))
                   .onErrorResumeNext(
                       throwable -> {
                         span.setStatus(StatusCode.ERROR, "Error in runLive Flowable execution");