fix(grpc): surface worker runtime exceptions

[KXH (KXHXK)]

Summary

• add an ignore_unhandled_exceptions option to GrpcWorkerAgentRuntime, matching the existing single-threaded runtime default
• retain the first unhandled event, read-loop, or background-task exception and raise it from stop() after cleanup when propagation is enabled
• replace done-callback re-raises with deterministic logging and lifecycle-based propagation
• add offline regression coverage for default logging, event-handler failures, read-loop failures, background-task failures, and first-exception preservation

Fixes #6140

Validation

• python -m pytest packages/autogen-ext/tests/test_worker_runtime.py --grpc -q (14 passed, 8 skipped)
• ruff check and ruff format --check for both changed files
• mypy for both changed files (Success: no issues found)
• git diff --check