feat(plugins): add on_agent_error_callback and on_run_error_callback lifecycle hooks

[STHITAPRAJNAS]

Summary

Fixes #4774

When an unhandled exception propagates out of an agent's _run_async_impl / _run_live_impl, or out of the runner's execution loop, the existing after_agent_callback / after_run_callback were silently skipped. This made fatal failures invisible to observability plugins (e.g. BigQuery analytics), inflating success rates and losing failure events entirely.

Changes

• BasePlugin: add on_agent_error_callback(agent, callback_context, error) and on_run_error_callback(invocation_context, error) with safe no-op defaults.
• PluginManager: add run_on_agent_error_callback / run_on_run_error_callback dispatch methods backed by a new _run_error_callbacks helper that fans out to all plugins (no early-exit) and logs — but does not propagate — individual plugin failures.
• base_agent.py: wrap run_async / run_live generator loops in try/except; call run_on_agent_error_callback before re-raising.
• runners.py: wrap the execute_fn generator loop in try/except; call run_on_run_error_callback before re-raising. after_run_callback is intentionally skipped on the error path so plugins can distinguish clean completions from fatal failures.

Design decisions

Decision	Rationale
Error callbacks are fire-and-forget (no early-exit, no return value)	They are pure observers — a broken plugin must not prevent others from recording the failure
after_run_callback is not called on error	Allows plugins to distinguish success from failure; analogous to how on_model_error_callback vs after_model_callback work
Individual plugin failures are logged, not re-raised	A broken observability plugin must never hide the original error
The original exception is always re-raised	The framework does not suppress errors; this is notification only

Testing plan

• tests/unittests/plugins/test_lifecycle_error_callbacks.py — 12 tests covering BasePlugin defaults, PluginManager fan-out semantics, argument forwarding, and no-early-exit behaviour
• tests/unittests/runners/test_runner_error_callbacks.py — 7 integration tests covering runner error/success paths with real InMemoryRunner
• tests/unittests/agents/test_agent_error_callbacks.py — 11 tests covering run_async and run_live error paths
• All 688 existing plugin + agent + runner tests pass unchanged

[rohityan]

Hi @STHITAPRAJNAS , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
Can you please fix the failing formatting tests. You can use autoformat.sh

[rohityan]

Hi @Jacksunwei , can you please review this.