Merge https://github.com/google/adk-python/pull/6600
**1\. Link to an existing issue (if applicable):**
- Related: google/adk-docs\#2045 (Addresses technical verification feedback in [https://github.com/google/adk-docs/pull/2045\#issuecomment-5184438669](https://github.com/google/adk-docs/pull/2045#issuecomment-5184438669))
**2\. Or, if no issue exists, describe the change:**
**Problem:**
1. Callable attribute bindings in `CloudEventAttributesBinding` were always evaluated against the event `payload`. This prevented developers from correlating CloudEvents with ADK runtime telemetry (such as session IDs or invocation IDs from `Context`).
2. Setting `time=OMIT` or `datacontenttype=OMIT` in `CloudEventAttributesBinding` skipped adding keyword arguments when calling `publish_message`. Because `publish_message` auto-generates default UTC timestamps and content types when arguments are `None` or omitted, `time=OMIT` generated a timestamp instead of omitting the header.
3. Sample READMEs omitted the required `pip install "google-adk[gcp]"` prerequisite step needed for Eventarc publishing.
**Solution:**
1. Added automatic signature inspection (`0-arg`, `1-arg`, and `2-arg` callables) to `CloudEventAttributesBinding` so callables can receive the event `payload`, the runtime `Context` (`tool_context`), or both, while preserving full backward compatibility with existing payload callbacks.
2. Setting `time=OMIT` or `datacontenttype=OMIT` now explicitly passes empty string (`""`) to `publish_message` so attributes are omitted from published CloudEvents. Explicitly setting required CloudEvent specification headers (`id=OMIT`, `specversion=OMIT`) now raises a `TypeError` at tool build time.
3. Updated sample agents and sample READMEs (`domain_specific_agent` and `generic_agent`) to demonstrate `Context` callables, `time=OMIT`, and GCP extra prerequisites.
### Testing Plan
**Unit Tests:**
- I have added or updated unit tests for my change.
- All unit tests pass locally.
Summary of passed `pytest` results:
```
uv run --all-extras pytest tests/unittests/integrations/eventarc -v
======================== 63 passed, 4 warnings, 13 subtests passed in 3.14s ========================
```
- Added `test_runtime_execution_with_context_and_payload_lambdas` to verify 1-parameter (`payload` or `Context`) and 2-parameter callables.
- Added `test_time_and_datacontenttype_omit_pass_empty_string` to verify omission of `time` and `datacontenttype`.
- Added `test_id_and_specversion_omit_raise_typeerror` to verify static validation against omitting mandatory CloudEvent specification headers.
**Manual End-to-End (E2E) Tests:**
- Verified that sample agent tools in `contributing/samples/integrations/eventarc/domain_specific_agent/agent.py` build and run correctly.
- Confirmed that `complete_outreach_lambda_tool` correctly injects `Context.session_id` into the event source and that `ping_system_tool` emits events without a timestamp header when configured with `time=OMIT`.
### Checklist
- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.
Additional context
Addresses the technical verification report on google/adk-docs#2045 comment.
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/6600 from milenvk:main 647744f65452358842982b4d88cd1401d4143c42
PiperOrigin-RevId: 964084439
Milen Kovachev
a3bd11152d