chore: import upstream snapshot with attribution

contributing/samples/context_management/migrate_session_db/README.md

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+# Loading and Upgrading Old Session Databases
+
+This example demonstrates how to upgrade a session database created with an older version of ADK to be compatible with the current version.
+
+## Sample Database
+
+This sample includes `dnd_sessions.db`, a database created with ADK v1.15.0. The following steps show how to run into a schema error and then resolve it using the migration script.
+
+## 1. Reproduce the Error
+
+First, copy the old database to `sessions.db`, which is the file the sample application expects.
+
+```bash
+cp dnd_sessions.db sessions.db
+python main.py
+```
+
+Running the application against the old database will fail with a schema mismatch error, as the `events` table is missing a column required by newer ADK versions:
+
+```
+sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such column: events.usage_metadata
+```
+
+## 2. Upgrade the Database Schema
+
+ADK provides a migration script to update the database schema. Run the following command to download and execute it.
+
+```bash
+# Clean up the previous run before executing the migration
+cp dnd_sessions.db sessions.db
+
+# Download and run the migration script
+curl -fsSL https://raw.githubusercontent.com/google/adk-python/main/scripts/db_migration.sh | sh -s -- "sqlite:///%(here)s/sessions.db" "google.adk.sessions.database_session_service"
+```
+
+This script uses `alembic` to compare the existing schema against the current model definition and automatically generates and applies the necessary migrations.
+
+**Note on generated files:**
+
+- The script will create an `alembic.ini` file and an `alembic/` directory. You must delete these before re-running the script.
+- The `sample-output` directory in this example contains a reference of the generated files for your inspection.
+- The `%(here)s` variable in the database URL is an `alembic` placeholder that refers to the current directory.
+
+## 3. Run the Agent Successfully
+
+With the database schema updated, the application can now load the session correctly.
+
+```bash
+python main.py
+```
+
+You should see output indicating that the old session was successfully loaded.
+
+## Limitations
+
+The migration script is designed to add new columns that have been introduced in newer ADK versions. It does not handle more complex schema changes, such as modifying a column's data type (e.g., from `int` to `string`) or altering the internal structure of stored data.