…lable executor (opensearch-project#6984)

FileReader.readLoop and its backpressure/batch logic branched on
System.currentTimeMillis(), so the max-read-time and drain-timeout tests
depended on wall-clock timing. On a slow or loaded runner the loop could
break before the first channel.read, leaving Mockito stubs unused and
failing with UnnecessaryStubbingException.

Inject a java.time.Clock into FileReader (public constructor defaults to
Clock.systemDefaultZone(), package-private constructor accepts a Clock),
matching the existing pattern in ServiceMapStatefulProcessor,
OTelApmServiceMapProcessor, and PipelineShutdown. Tests now drive time
with a mocked Clock: the read stub advances the fake clock past the
deadline after delivering data once, so exactly one read happens
deterministically. Removes all Thread.sleep and wall-clock assumptions
from these tests.

FileReaderPoolTest bookkeeping tests submitted readers to a real thread
pool where the reader completed on a background thread and mutated the
active/pending counts before the assertions ran. Add
createPoolWithNonRunningExecutor so queueing, promotion, and close logic
run synchronously. Tests that verify execution keep the real executor
and await via verify(..., timeout(...)).

Signed-off-by: Srikanth Padakanti <srikanth_padakanti@apple.com>