fix: run on_event callbacks for before_run early exits

[jaywang172]

Link to Issue or Description of Change

• Fixes before_run early-exit events bypass on_event_callback #7007

Problem:

before_run_callback may halt a run by returning types.Content. The runner
converts that content into an event, but the early-exit paths currently persist
and yield the event without invoking on_event_callback.

This affects legacy agent execution, node / LlmAgent execution, and live
execution. It means event plugins used for logging, auditing, redaction, or
metadata enrichment do not observe these runner-produced responses.

Solution:

Route early-exit events through the same private event-processing step used by
normal runner events:

metadata → on_event_callback → merge

before persistence and yielding. Keeping this processing in a private Runner
helper prevents the normal and early-exit paths from drifting while introducing
no public API.

Before:

before_run → early-exit Event → persist / yield

After:

before_run → early-exit Event → on_event_callback → merge → persist / yield

Persistence eligibility remains based on the original event, preserving the
existing live-event persistence policy.

Testing Plan

Unit Tests:

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

Added a parameterized regression test for legacy async, node async, and live
runner execution. It verifies that:

• on_event_callback is invoked for the early-exit event;
• callback modifications appear in the yielded event;
• the same modifications appear in the persisted session event;
• existing event merge semantics are preserved.

Validation:

• related runner/plugin tests: 164 passed;
• regression tests: passed on Python 3.10, 3.11, 3.12, 3.13, and 3.14;
• full unit suite on current main (c7ffcfa8): 13,959 passed,
  85 skipped, 27 xfailed, 2 xpassed, and 24 subtests passed;
• pre-commit: passed;
• git diff --check: passed;
• wheel and sdist builds: passed.

No Google API key or Application Default Credentials are required.

Manual End-to-End (E2E) Tests:

Ran a local Runner smoke test with InMemorySessionService. A plugin returned
content from before_run_callback, replaced the synthesized event in
on_event_callback, and the script verified that the processed event was both
yielded and persisted. The early exit prevented any model request.

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 dependencies).

Additional context

This is a focused runtime lifecycle fix. It does not change plugin ordering,
event merge behavior, or any public API.