feat(eventarc): Support Context callables and correct OMIT behavior

[milenvk]

1. Link to an existing issue (if applicable):

• Related: docs: add Eventarc tool integration page and code snippets adk-docs#2045 (Addresses technical verification feedback in docs: add Eventarc tool integration page and code snippets adk-docs#2045 (comment))

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

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Addresses the technical verification report on google/adk-docs#2045 comment.