feat(cli): Implement save-session-on-runtime feature for periodic session checkpointing

This commit introduces the --save_session_on_runtime feature for the ADK
CLI, enabling periodic session checkpointing during interactive agent
sessions.

Feature Overview

When enabled with --save-session-on-runtime, the agent session state is
automatically saved to disk every 60 seconds during interactive
sessions.

On subsequent runs with the same agent, the last checkpoint is
automatically loaded instead of starting a fresh session.

Implementation Details

• Session data saved includes:

  • Conversation history
  • Tool call logs
  • Agent memory state
  • Turn count

• Sessions are stored in:

  {agent_directory}/.adk/runtime_session.json
  

• Uses asyncio background tasks to avoid blocking the interactive
  session

• Includes proper error handling to prevent disrupting the user
  experience

• Automatically creates the .adk directory if it does not exist

Configuration

• Enabled via:

  adk run --save_session_on_runtime
  

Files Modified

• src/google/adk/cli/cli.py
  Core implementation in run_interactively and run_cli

• src/google/adk/cli/cli_tools_click.py
  CLI option definition and parameter wiring

Unit Tests

Added: - tests/unittests/cli/test_save_session_on_runtime.py

Test Coverage

• Verifies CLI help includes the new option
• Confirms flag is properly passed from CLI command to internal
  functions
• Validates run_cli correctly forwards save_session_on_runtime to
  run_interactively
• Tests session loading logic for runtime session restoration
• Covers error handling for corrupted or missing runtime session files

Test Status

• Unit tests pass locally
• Syntax validation successful

closes : #5466