Do not fail the process collector when the limits file has no Max open file line

[PerryLink]

What

ProcessCollector.collect() reads /proc/<pid>/limits and only assigns max_fds when it finds the
Max open file line, but then uses it unconditionally:

for line in limits:
    if line.startswith(b'Max open file'):
        max_fds = GaugeMetricFamily(...)
        break
open_fds = GaugeMetricFamily(...)
result.extend([open_fds, max_fds])     # UnboundLocalError if the line was absent

If the file opens but the line is not there — seen in the wild with a corrupted Max open fides
header (home-assistant/core#140045, traceback at process_collector.py:93) — the collector raises
UnboundLocalError, and because PROCESS_COLLECTOR is registered in the default REGISTRY with
auto_describe=True, /metrics itself fails.

The change

Bind max_fds to None before the loop, append it only when it was found, and keep the existing
OSError guard as-is. No new validation: the metric is simply omitted when the kernel does not
report it, which is what happens on every platform without /proc already.

Adds a regression test using the repository's existing fixture convention, with a limits file that
has no Max open file line.

Verification

• before: python -m unittest tests.test_process_collector → UnboundLocalError: cannot access local variable 'max_fds' at process_collector.py:93 (reproduced on Windows/Python 3.12.14 and on
  WSL2/Linux/Python 3.12.3);
• after: 5 tests OK on both;
• full suite: 428 passed before, 429 passed after; flake8 and isort clean; mypy reports only the
  pre-existing Windows-only warnings that the unmodified file reports too.

The adjacent float('unlimited') issue (#80/#383) is deliberately not bundled here.

DCO: commit is signed off.