test: add mutation test case

[cojenco]

Context: mutation test caught missing test case in internal cl/609739391

Not sure why productive coverage check is angry as there is test coverage through

storage-testbench/tests/test_grpc_server.py

Lines 2113 to 2151 in a60a714

	def test_bidi_write_object_resumable(self):
	start = self.grpc.StartResumableWrite(
	storage_pb2.StartResumableWriteRequest(
	write_object_spec=storage_pb2.WriteObjectSpec(
	resource=storage_pb2.Object(
	name="object-name", bucket="projects/_/buckets/bucket-name"
	)
	)
	),
	context=unittest.mock.MagicMock(),
	)
	self.assertIsNotNone(start.upload_id)
	QUANTUM = 256 * 1024
	media = TestGrpc._create_block(2 * QUANTUM + QUANTUM / 2).encode("utf-8")
	offset = 0
	content = media[0:QUANTUM]
	r1 = storage_pb2.BidiWriteObjectRequest(
	upload_id=start.upload_id,
	write_offset=offset,
	checksummed_data=storage_pb2.ChecksummedData(
	content=content, crc32c=crc32c.crc32c(content)
	),
	flush=True,
	finish_write=False,
	)
	offset = QUANTUM
	content = media[QUANTUM : 2 * QUANTUM]
	r2 = storage_pb2.BidiWriteObjectRequest(
	upload_id=start.upload_id,
	write_offset=offset,
	checksummed_data=storage_pb2.ChecksummedData(
	content=content, crc32c=crc32c.crc32c(content)
	),
	flush=True,
	state_lookup=True,
	finish_write=False,
	)
	streamer = self.grpc.BidiWriteObject([r1, r2], "fake-context")

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (a60a714) to head (cd5d13b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
+ Coverage   98.63%   98.68%   +0.05%     
==========================================
  Files          50       50              
  Lines        8365     8462      +97     
==========================================
+ Hits         8251     8351     +100     
+ Misses        114      111       -3     

Flag	Coverage Δ
unittests	98.68% <100.00%> (+0.05%)	⬆️

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