enh: reduce per-task overhead

[satra]

closes #850

Five targeted optimisations identified via cProfile on the audio-file benchmark from issue #850:

1. Etelemetry sentinel (job.py, submitter.py): Replace the None initial value of Job._etelemetry_version_data with a distinct _ETELEMETRY_UNCHECKED sentinel. Previously a failed network check returned None, keeping the is None guard True forever and re-issuing the HTTP request on every task (~93 ms each).

2. Function bytes cache (hash.py): Add a module-level _function_bytes_cache keyed by (module, qualname, mtime_ns). inspect.getsource() + ast.parse() now runs at most once per function per session; subsequent calls cost only a single os.stat().

3. Skip hash-change check for non-FileSet tasks (job.py): Call TypeParser.contains_type(FileSet, ...) on each input field; if none match, skip the expensive full hash recomputation in _check_for_hash_changes(). Scalar/pure-Python values cannot mutate under Pydra.

4. In-memory result cache (job.py): Store the completed Result on self._cached_result at the end of run() so same-process callers (e.g. Submitter.__call__ with DebugWorker) do not need to deserialise it back from disk. The field is excluded from __getstate__ so subprocess/Slurm workers still use the disk path.

5. Once-per-location PersistentCache.clean_up() (hash.py): Track which cache locations have already been scanned in a class-level set (_session_cleanups_done). The O(n) iterdir() + stat() loop no longer runs after every task. path.unlink(missing_ok=True) makes concurrent cleanup by multiple Slurm nodes on shared NFS safe.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.58%. Comparing base (876be56) to head (cab9af2).

Files with missing lines	Patch %	Lines
pydra/utils/hash.py	67.34%	9 Missing and 7 partials ⚠️
pydra/engine/job.py	84.61%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (71.42%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (876be56) and HEAD (cab9af2). Click for more details.

HEAD has 46 uploads less than BASE

Flag	BASE (876be56)	HEAD (cab9af2)
	52	6

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #858       +/-   ##
===========================================
- Coverage   88.35%   34.58%   -53.77%     
===========================================
  Files          88       66       -22     
  Lines       18198    11316     -6882     
  Branches     3565     1504     -2061     
===========================================
- Hits        16079     3914    -12165     
- Misses       1737     6975     +5238     
- Partials      382      427       +45     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.