feat(durable): Add durable session persistence layer for long-horizon agents

This PR implements a durable session persistence layer for ADK, enabling
cross-process checkpoint-based recovery for long-running agent tasks.

## Key Features

- **DurableSessionConfig**: Configuration for durable cross-process checkpointing
- **BigQueryCheckpointStore**: Two-phase commit checkpoint storage (BQ metadata + GCS blobs)
- **CheckpointableAgentState**: Abstract interface for agents supporting durability
- **WorkspaceSnapshotter**: GCS-based workspace directory snapshotting

## Implementation Details

- Two-phase commit: GCS blob upload → BigQuery metadata insert
- SHA-256 checkpoint integrity verification
- Lease-based concurrency control for safe resume
- Async-first API design for non-blocking I/O

## Demo

A fully functional demo is deployed on Cloud Run showcasing:
- Real-time checkpoint visualization
- Task failure simulation and recovery
- BigQuery metadata queries
- Final task output display

Demo URL: https://durable-demo-201486563047.us-central1.run.app

## Files Added

- src/google/adk/durable/ - Core durable module
- contributing/samples/long_running_task/ - Demo agent and UI
- tests/unittests/durable/ - Unit tests

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: caohy1988

contributing/samples/long_running_task/README.md

@@ -0,0 +1,182 @@
+# Durable Session Demo
+
+This demo showcases the durable session persistence feature in ADK, which
+enables checkpoint-based durability for long-running agent invocations.
+
+## Overview
+
+Durable sessions provide:
+- **Checkpoint persistence**: Agent state is saved to BigQuery + GCS
+- **Failure recovery**: Resume from the last checkpoint after crashes
+- **Host migration**: Move sessions between hosts seamlessly
+- **Lease management**: Prevent concurrent modifications
+
+## Prerequisites
+
+1. **Google Cloud Project** with billing enabled
+2. **APIs enabled**:
+   - BigQuery API
+   - Cloud Storage API
+   - Vertex AI API (for Gemini models)
+3. **IAM permissions**:
+   - `roles/bigquery.dataEditor`
+   - `roles/storage.objectAdmin`
+   - `roles/aiplatform.user`
+
+## Setup
+
+### 1. Configure your environment
+
+```bash
+# Set your project
+export PROJECT_ID="test-project-0728-467323"
+gcloud config set project $PROJECT_ID
+
+# Set your Google Cloud API key (required for Gemini 3)
+export GOOGLE_CLOUD_API_KEY="your-api-key-here"
+
+# Authenticate
+gcloud auth application-default login
+```
+
+### 2. Create BigQuery and GCS resources
+
+```bash
+# Run the setup script
+python contributing/samples/long_running_task/setup.py
+
+# To verify setup
+python contributing/samples/long_running_task/setup.py --verify
+
+# To clean up resources
+python contributing/samples/long_running_task/setup.py --cleanup
+```
+
+### 3. Run the demo
+
+```bash
+adk web contributing/samples/long_running_task
+```
+
+## Demo Scenarios
+
+### Scenario 1: Long-running table scan
+
+```
+User: Scan the bigquery-public-data.samples.shakespeare table
+
+Agent: [Calls simulate_long_running_scan]
+       [Checkpoint written at async boundary]
+       [Scan completes after ~5-10 seconds]
+       The scan found 164,656 rows with the following findings:
+       - Found 5 instances of 'to be or not to be'
+       - Most common word: 'the' (27,801 occurrences)
+       - Unique words: 29,066
+```
+
+### Scenario 2: Multi-stage pipeline
+
+```
+User: Run a pipeline from source_table to dest_table with transformations:
+      filter, aggregate, join
+
+Agent: [Calls run_data_pipeline]
+       [Checkpoint written at each stage boundary]
+       Pipeline completed successfully:
+       - Stage 1 (filter): 45,000 rows processed
+       - Stage 2 (aggregate): 32,000 rows processed
+       - Stage 3 (join): 28,000 rows processed
+```
+
+### Scenario 3: Failure recovery
+
+1. Start a long-running scan
+2. Kill the process mid-execution
+3. Restart and resume with the invocation_id
+4. Agent continues from the last checkpoint
+
+## Architecture
+
+```
+                    +-----------------+
+                    |     Agent       |
+                    |  (LlmAgent)     |
+                    +--------+--------+
+                             |
+                             v
+                    +-----------------+
+                    |     Runner      |
+                    | (with durability)|
+                    +--------+--------+
+                             |
+            +----------------+----------------+
+            |                                 |
+            v                                 v
+    +--------------+                 +----------------+
+    |   BigQuery   |                 |      GCS       |
+    |  (metadata)  |                 | (state blobs)  |
+    +--------------+                 +----------------+
+    | - sessions   |                 | - checkpoints/ |
+    | - checkpoints|                 |   {session_id}/|
+    +--------------+                 +----------------+
+```
+
+## Configuration
+
+The agent is configured in `agent.py`:
+
+```python
+app = App(
+    name="durable_session_demo",
+    root_agent=root_agent,
+    resumability_config=ResumabilityConfig(is_resumable=True),
+    durable_session_config=DurableSessionConfig(
+        is_durable=True,
+        checkpoint_policy="async_boundary",
+        checkpoint_store=BigQueryCheckpointStore(
+            project=PROJECT_ID,
+            dataset=DATASET,
+            gcs_bucket=GCS_BUCKET,
+        ),
+        lease_timeout_seconds=300,
+    ),
+)
+```
+
+### Checkpoint Policies
+
+- `async_boundary`: Checkpoint when hitting async/long-running operations
+- `every_turn`: Checkpoint after every agent turn
+- `manual`: Only checkpoint when explicitly requested
+
+## Monitoring
+
+### View sessions
+
+```sql
+SELECT * FROM `test-project-0728-467323.adk_metadata.sessions`
+ORDER BY updated_at DESC
+LIMIT 10;
+```
+
+### View checkpoints
+
+```sql
+SELECT * FROM `test-project-0728-467323.adk_metadata.checkpoints`
+ORDER BY created_at DESC
+LIMIT 10;
+```
+
+### List checkpoint blobs
+
+```bash
+gsutil ls -l gs://test-project-0728-467323-adk-checkpoints/checkpoints/
+```
+
+## Cleanup
+
+To remove all resources created by this demo:
+
+```bash
+python contributing/samples/long_running_task/setup.py --cleanup
+```