fix(events): exclude state-only events from final responses

[Epsilon006]

Link to Issue or Description of Change

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

• Closes: A before_agent_callback which changes the state results in an empty Event being returned #7078

Problem:
A before_agent_callback or after_agent_callback that only mutates state
causes BaseAgent to emit a content-less event carrying the state delta. The
default branch of Event.is_final_response() currently classifies that event
as final, so consumers can finish the turn before the actual agent response.

Solution:
Treat events with no content or structured output and a non-empty
state_delta as non-final. This reuses the event's existing shape instead of
adding framework-event metadata. Empty Content(parts=[]) events remain final,
preserving legitimate content-less model completion semantics.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added regression coverage for state-only events, empty Content compatibility,
and both before/after agent callback state-delta paths.

Relevant tests, including A2A converters/executor, pass on every supported
Python version:

Python 3.10: 227 passed, 2 skipped
Python 3.11: 227 passed, 2 skipped
Python 3.12: 227 passed, 2 skipped
Python 3.13: 227 passed, 2 skipped
Python 3.14: 227 passed, 2 skipped

pre-commit run --files passes all applicable hooks. Full tox was also run;
the Windows host has unrelated existing platform/environment failures (GBK
encoding, Unix shell/path assumptions, network-dependent tests, and telemetry
snapshots). None of those failures are in the changed files or relevant test
set.

Manual End-to-End (E2E) Tests:

Ran a deterministic agent through InMemoryRunner. Its
before_agent_callback sets callback_state, then the agent emits its response.
The state-only event is observable and persisted but is not final; the following
agent response is final.

event[0]: content=None, state_delta={'callback_state': 'updated'}, is_final_response=False
event[1]: content='actual agent response', state_delta={}, is_final_response=True
persisted_state={'callback_state': 'updated'}
RESULT: PASS

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 (N/A; no dependent changes).

Additional context

The patch intentionally does not add a new public event field. Existing
priority semantics for terminal errors and skip_summarization remain
unchanged, and plain empty events retain their historical final-response
behavior.

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