Reject nonexistent requested paths during asset discovery

[AtomicGlance]

Fixes #1094.
find_dandi_files() currently allows nonexistent paths to be silently ignored or represented as assets. This change checks every requested root before yielding any files and raises FileNotFoundError with the offending filename.
The check uses lstat() so broken symlinks remain discoverable. This preserves DANDI’s missing-content handling for unfetched DataLad files. Existing empty directories still return no assets, and no additional recursive walk is introduced.
Tests cover missing paths with recognized, unrecognized, and hidden names; both allow_all settings; multiple requested roots; empty directories; and broken symlinks. A CLI regression test also confirms that dandi validate retains its existing missing-path error.
Validation on Windows with Python 3.13:

• File tests excluding upload integration tests: 28 passed, 1 skipped.
• Focused discovery and CLI tests: 12 passed, 1 skipped.
• Black, isort, flake8, and mypy dandi passed.

[codecov]

Codecov Report

❌ Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (ad8f64d) to head (5fcb0d6).

Files with missing lines	Patch %	Lines
dandi/tests/test_files.py	85.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1932      +/-   ##
==========================================
+ Coverage   90.77%   90.83%   +0.05%     
==========================================
  Files          92       92              
  Lines       14130    14167      +37     
==========================================
+ Hits        12826    12868      +42     
+ Misses       1304     1299       -5     

Flag	Coverage Δ
unittests	90.83% <89.18%> (+0.05%)	⬆️

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[AtomicGlance]

This PR is ready for review, but GitHub won't let me remove its draft status, through either the website or CLI. Could you please mark it ready and add the patch label? All other checks are passing. Thank you!

[AtomicGlance]

This PR is ready for review, but GitHub won't let me remove its draft status, through either the website or CLI. Could you please mark it ready and add the patch label? All other checks are passing. Thank you!

@yarikoptic Hi, can you have a look at this?

[AtomicGlance]

Hi @yarikoptic, thanks again for the review! Could you please rerun the failed and cancelled jobs in the latest Tests workflow, and the cancelled label check, when you have a chance? Both failed Windows jobs report that the hosted runner lost communication with GitHub; neither provides a test failure traceback. I tried requesting a rerun, but GitHub returned a 403 because it requires repository admin rights.

The explicit lexists() fix is pushed and the review conversation is addressed. Thanks for your help!

[AtomicGlance]

Hi @yarikoptic, a gentle follow-up when you have a moment: could you please rerun the failed and cancelled jobs in the Tests workflow and the cancelled label check? The latest results are still from October 5. Both failed Windows jobs reported a runner disconnection; your requested lexists() change is in place. Once the rerun finishes, I'd appreciate another look at the PR.

My related Archive fix, dandi/dandi-archive#2914, is also waiting for review. Only the deployment-preview check is reported there, so I can't claim the backend suite is green. If someone else is better placed to review it or enable the backend tests, could you point me their way?

Thanks again for your time and guidance!

[yarikoptic]

clicking "Update branch". Next time if you need rerun -- just make sure it is up to date with current master and/or rewrite/force push last commit.