feat: Add API endpoint to cancel in-progress agent tasks

[Oxygen56]

Summary

Adds a POST /apps/{app}/users/{user}/sessions/{session}:cancel endpoint that sets temp:cancelled in the session state via EventActions. The agent checks this flag at key execution points (before LLM calls, before tool execution) and gracefully halts when cancellation is detected.

Changes

• api_server.py: New /cancel endpoint with FastAPI
• base_llm_flow.py: _is_session_cancelled() check + early return in _call_llm_async()
• functions.py: Cancellation check before tool execution
• tests/unittests/cli/test_cancel_session.py: 12 unit tests

Testing Plan

pytest tests/unittests/cli/test_cancel_session.py -v

Tests cover:

• _is_session_cancelled with no session / no state / no flag / flag true / flag false
• Cancellation detection in _call_llm_async flow
• Cancel event construction with temp:cancelled state delta
• Cancel response format validation

🤖 Generated with Claude Code
Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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[adk-bot]

Response from ADK Triaging Agent

Hello @Oxygen56, thank you for creating this PR to add the session cancellation API! This is a very useful feature.

To help us review and merge your contribution, please address the following items according to our Contribution Guidelines:

1. Contributor License Agreement (CLA):

   • It looks like the CLA check has failed. Please sign the Google Contributor License Agreement so we can officially accept your changes. You can visit https://cla.developers.google.com/ to sign it or check your current agreements.

2. Unit Tests:

   • Our guidelines require all code changes to include comprehensive tests to prevent regressions. Please add unit tests covering the new endpoint and cancellation behavior (e.g., under tests/unittests/).

3. Manual End-to-End (E2E) Tests & Evidence:

   • For new features, please include a testing plan section in your PR description.
   • Please also provide manual E2E test evidence (such as console logs or screenshots) showing the cancel endpoint being called and the execution halting gracefully as expected.

This information will help our reviewers better understand and verify your fix more efficiently. Thank you!

[rohityan]

Hi @Oxygen56 , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!