fix: improve file naming logic in get_next_available_path

[fdrgsp]

Update the get_next_available_path function...I found it weird that when requesting test.ome.tiff in a folder that had no test*.ome.tiff files (but contained other unrelated .ome.tiff files (e.g., other_009.ome.tiff)) the result was test_010.ome.tiff instead of the expected test.ome.tiff.

The issue was that the glob pattern *.ome.tiff matched all .ome.tiff files in the directory, not just those sharing the same stem. This caused unrelated files' counters to influence the result. Fixed by scoping the glob to {stem}*.ome.tiff.

---

New behavior:

Basic: no existing files in directory

Requested path	Result	Why
test.ome.tiff	test.ome.tiff	Doesn't exist, no conflicts
test_001.ome.tiff	test_001.ome.tiff	Doesn't exist, no conflicts
test_02.ome.tiff	test_02.ome.tiff	Doesn't exist, no conflicts

test.ome.tiff exists in directory

Requested path	Result	Why
test.ome.tiff	test_001.ome.tiff	Next available counter

test.ome.tiff and test_004.ome.tiff exist

Requested path	Result	Why
test.ome.tiff	test_005.ome.tiff	Next after highest counter
test_003.ome.tiff	test_005.ome.tiff	Requested counter < max, uses max+1
test_010.ome.tiff	test_010.ome.tiff	Requested counter > max, keeps it
test_02.ome.tiff	test_02.ome.tiff	_02 is not a counter (< 3 digits), treated as distinct stem, no test_02* files exist

Bug fix: other_009.ome.tiff exists (unrelated file)

Requested path	Old (broken) result	New (fixed) result	Why
test.ome.tiff	test_010.ome.tiff	test.ome.tiff	Old glob *.ome.tiff matched unrelated files; new glob test*.ome.tiff scopes to same stem
test.ome.tiff (after creating it)	test_010.ome.tiff	test_001.ome.tiff	Counter now based only on test_* files

test_999.ome.tiff exists (digit overflow)

Requested path	Result	Why
test.ome.tiff	test_1000.ome.tiff	Grows beyond 3 digits when needed

test_12345.ome.tiff exists (high counter)

Requested path	Result	Why
test_12345.ome.tiff	test_12346.ome.tiff	Preserves digit count from existing files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.87%. Comparing base (12508e3) to head (c0da02f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #529   +/-   ##
=======================================
  Coverage   86.87%   86.87%           
=======================================
  Files         115      115           
  Lines       14093    14097    +4     
=======================================
+ Hits        12243    12247    +4     
  Misses       1850     1850           

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