Accept broken symlinks as path arguments of dandi validate

[CodyCBakerPhD]

Another pre-PR splintered out of #1933 since it does actually affect the CLI today (granted, IDK why someone WOULD use the CLI in that way today on a datalad dataset, but they COULD)

dandi validate already handles annexed files whose content has not been fetched (the broken symlinks of a DataLad dataset) according to --missing-file-content: error, skip, or only-non-data. Those policies were applied only when such a file was reached through its directory, though: given directly on the command line, the file never got past argument parsing, because click.Path(exists=True) follows the link and rejected it with "does not exist". Check the path with lexists() instead, so that

dandi validate --missing-file-content=skip sub-01/sub-01.nwb

works the same as validating sub-01/, while paths that truly do not exist are still rejected with the same error.

Claude-Session: https://claude.ai/code/session_01HFhMPs6vF5zyeHDvPTvRcA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.33%. Comparing base (b294fc9) to head (4f316f7).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           claude/docs-cli-options-sync    #1937      +/-   ##
================================================================
+ Coverage                         78.30%   78.33%   +0.02%     
================================================================
  Files                                92       92              
  Lines                             14068    14087      +19     
================================================================
+ Hits                              11016    11035      +19     
  Misses                             3052     3052              

Flag	Coverage Δ
unittests	78.33% <100.00%> (+0.02%)	⬆️

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