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 88.57143% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.34%. Comparing base (c3931cc) to head (63ee56a).

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   78.29%   78.34%   +0.04%     
==========================================
  Files          92       92              
  Lines       14068    14103      +35     
==========================================
+ Hits        11015    11049      +34     
- Misses       3053     3054       +1     

Flag	Coverage Δ
unittests	78.34% <88.57%> (+0.04%)	⬆️

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