fs: only back Blobs with regular files

[xia-chao]

A file-backed Blob is supposed to read the given file, but FdEntry::Create only checked that stat succeeded. So openAsBlob returned a Blob for a directory, with the size of the directory, and reading it failed later with NotReadableError and code 0, which hides the real EISDIR.

A fifo was worse. openAsBlobSync returned a zero size Blob right away, and the first read then blocked the main thread. ReaderImpl::Create opens the path synchronously, and a fifo with no writer never completes that open. Timers and abort signals never run, so the process just hangs. I hit this with a probe that prints a heartbeat every 50 ms; the heartbeat never fired and the probe had to be killed with timeout.

The fix checks the stat mode and rejects anything that is not a regular file, so directories report EISDIR and everything else EINVAL, through the same UVException that #65517 added for missing paths. The TODO on FdEntry already said this entry should only allow regular files.

The test covers a directory and a fifo.

Refs #65514, where the directory case was left out on purpose as a separate behavior change.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (addcba2) to head (dece64d).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dataqueue/queue.cc	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66204      +/-   ##
==========================================
- Coverage   90.29%   90.27%   -0.02%     
==========================================
  Files         790      790              
  Lines      272529   272535       +6     
  Branches    52043    52033      -10     
==========================================
- Hits       246079   246040      -39     
- Misses      16906    16954      +48     
+ Partials     9544     9541       -3     

Files with missing lines	Coverage Δ
src/dataqueue/queue.cc	66.05% <75.00%> (+0.06%)	⬆️

... and 33 files with indirect coverage changes

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