fix(providers/apache/beam): use selectors in run_beam_command - #74433

What happens

When you run a Beam pipeline operator — BeamRunPythonPipelineOperator,
BeamRunJavaPipelineOperator, BeamRunGoPipelineOperator, or a Google Dataflow
operator that launches through the shared Beam hook — the task launches the
pipeline as a subprocess and streams its stdout/stderr back to the task log.

That streaming is done in run_beam_command, which waits for output using
Python's stdlib select.select():

reads = [proc.stderr, proc.stdout]
readable_fds, _, _ = select.select(reads, [], [], 5)

select.select() is backed by a fixed-size fd_set bitmap of FD_SETSIZE (1024)
bits. It physically cannot represent a file descriptor whose number is >= 1024,
and raises "ValueError: filedescriptor out of range in select()" when asked to.
This ceiling is compile-time; it is unrelated to RLIMIT_NOFILE, so raising
ulimit -n does not help.

So whether this fires depends only on the fd NUMBER assigned to the subprocess
pipes. File descriptors are handed out lowest-free-first, so when the task
process already holds many descriptors open, the Beam subprocess's stdout/stderr
pipes are assigned numbers >= 1024 — and the pipeline run aborts with the error
above, even though nothing is actually wrong with the pipeline.

This is the exact same root cause and class of fix already accepted for the SSH
provider in #74211 (hook), and it traces back to the same
FD_SETSIZE issue I originally reported in #74205. The synchronous Beam path is
affected; the async path (run_beam_command_async, asyncio-based) is not.

Fix

Replace the select.select() loop with selectors.DefaultSelector (epoll/poll),
which has no FD_SETSIZE ceiling. Behaviour is otherwise unchanged.

Reproduction / validation

• Reproduced on the real run_beam_command with a real subprocess while the
  process held 1,100+ open fds: the old select() code raises
  "filedescriptor out of range in select()"; the fix captures the subprocess
  output correctly.
• With few fds open (< 1024) both behave identically — the only variable that
  flips pass/fail is the fd number.
• Updated the existing logging test to patch selectors; added a regression test
  that runs a real subprocess under a high fd count (fails on old select(),
  passes with selectors). ruff check + format clean.

related: #74205