Prevent scheduler crash when process/thread missing from log format

[anxkhn]

When a log record reaches the percent formatter without callsite information, for
example a standard-library warning routed through Airflow's logging bridge, the
process and thread fields are absent. The formatter fell back to the
"(unknown)" string for every missing callsite field, so a log format using
the numeric %(process)d or %(thread)d specifiers (the default console
format renders a process-id column) raised
TypeError: %d format: a real number is required, not str and could take down
the scheduler at startup.

This gives those two numeric callsite parameters a numeric fallback of 0, the
same way lineno is already handled a few lines above, so the format string
never receives a string where a number is expected. The "(unknown)" string
fallback is left intact for the text callsite parameters
(pathname/module/threadName/processName), which are rendered with
%s.

A parametrized regression test formats %(process)d:%(thread)d for a record
with no callsite info (the exact issue reproduction) and for one where those keys
are explicitly None, asserting it renders 0:0 instead of raising; it fails
with the TypeError before the change and passes after.

closes: #66195

---

Was generative AI tooling used to co-author this PR?

• Yes

Final diff (2 files, +24/-0)

1. shared/logging/src/airflow_shared/logging/percent_formatter.py (+6): in
   _LazyLogRecordDict.__getitem__, add
   if key == "process" or key == "thread": return self.event.get(key) or 0
   immediately after the existing lineno numeric-fallback case, with a short
   comment explaining why (numeric %d params must not fall back to the
   "(unknown)" string used for text params).
2. shared/logging/tests/logging/test_percent_formatter.py (+18): add
   test_numeric_callsite_without_process_or_thread, parametrized missing
   (no callsite keys, the issue repro) and none (keys present but None), in
   the existing TestPercentFormatRender class next to test_no_callsite; adds
   the import pytest it needs.

[potiuk]

Nice!