feat(logging)!: Use StructuredLogHandler for Cloud Run Jobs by default

[fajpunk]

BREAKING CHANGE: Use StructuredLogHandler instead of CloudLoggingHandler by default in Cloud Run Jobs. There's no reason that I can think that we can't just log to stdout in Cloud Run Jobs by default like we do in Cloud Run Services and Cloud Run Functions. Starting a background thread for log shipping adds complexity and can cause errors, especiallly when interactiing with other libs like aiohttp, leading to errors like this:

  File "/usr/local/lib/python3.13/site-packages/aiohttp/connector.py", line 379, in __del__
    self._loop.call_exception_handler(context)
  File "/usr/local/lib/python3.13/asyncio/base_events.py", line 1925, in call_exception_handler
    logger.error('Exception in default exception handler',
  File "/usr/local/lib/python3.13/logging/__init__.py", line 1554, in error
    self._log(ERROR, msg, args, **kwargs)
  File "/usr/local/lib/python3.13/logging/__init__.py", line 1670, in _log
    self.handle(record)
  File "/usr/local/lib/python3.13/logging/__init__.py", line 1686, in handle
    self.callHandlers(record)
  File "/usr/local/lib/python3.13/logging/__init__.py", line 1746, in callHandlers
    hdlr.handle(record)
  File "/usr/local/lib/python3.13/logging/__init__.py", line 1027, in handle
    self.emit(record)
  File "/usr/local/lib/python3.13/site-packages/google/cloud/logging_v2/handlers/handlers.py", line 218, in emit
    self.transport = self._transport_cls(
  File "/usr/local/lib/python3.13/site-packages/google/cloud/logging_v2/handlers/transports/background_thread.py", line 324, in __init__
    self.worker.start()
  File "/usr/local/lib/python3.13/site-packages/google/cloud/logging_v2/handlers/transports/background_thread.py", line 173, in start
    self._thread.start()
  File "/usr/local/lib/python3.13/threading.py", line 976, in start
    _start_joinable_thread(self._bootstrap, handle=self._handle,
PythonFinalizationError: can't create new thread at interpreter shutdown

Of course just because I can't think of a reason why CloudLoggingHandler should be the default doesn't mean there isn't one, and this is a breaking change that could cause problems for folks depending on the existing behavior.

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Fixes #15407 🦕

[gemini-code-assist]

Code Review

This pull request adds support for Cloud Run jobs (cloud_run_job) to the default logging handler, returning a StructuredLogHandler when detected, and includes a corresponding unit test. The feedback suggests combining the duplicate elif blocks for Cloud Run revisions and jobs into a single membership check, and updating the mocked environment variables in the unit test to use string values instead of integers to accurately mimic os.environ.

[gemini-code-assist]

Instead of having separate elif blocks that return the exact same StructuredLogHandler instance, we can combine them into a single elif block using membership testing (in). This reduces duplication and improves maintainability.

Suggested change

	elif monitored_resource.type == _RUN_RESOURCE_TYPE:
	return StructuredLogHandler(**kw, project_id=self.project)
	elif monitored_resource.type == _JOB_RESOURCE_TYPE:
	return StructuredLogHandler(**kw, project_id=self.project)
	elif monitored_resource.type in (_RUN_RESOURCE_TYPE, _JOB_RESOURCE_TYPE):
	return StructuredLogHandler(**kw, project_id=self.project)