Add file type validation

[spider-yamet]

What problem does this PR solve?

This PR fixes WebDAV sync behavior for unsupported file types (#13795).

Previously, the WebDAV connector selected files primarily by modified time (and size threshold) and could still pass unsupported extensions into the download/document-generation path. This caused unnecessary processing and inconsistent behavior compared with connectors that validate file type earlier.

This change adds extension validation in two places:

1. Early filter during recursive listing to skip unsupported files before they enter the download flow.
2. Defensive filter before download/document creation to prevent unsupported files from being processed if any listing edge case slips through.

It also wires allow_images into the WebDAV sync path so image extension handling follows connector policy.

Scope is intentionally limited to WebDAV for a focused bug-fix PR.

Type of change

• Bug Fix (non-breaking change which fixes an issue)

How was this tested?

• Manual verification with mixed file types under the configured WebDAV path:
  • supported: .pdf, .txt, .md
  • unsupported: .exe, .bin, .dat
• Triggered full sync and polling sync.
• Confirmed unsupported files are skipped before download.
• Confirmed supported files are still indexed normally.
• Confirmed image handling follows allow_images setting.

Fixes: #13795

[spider-yamet]

@yingfeng Would love to hear your opinion on this PR. Thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.72%. Comparing base (cb78ce0) to head (94cabba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13802      +/-   ##
==========================================
- Coverage   98.14%   96.72%   -1.43%     
==========================================
  Files          10       10              
  Lines         702      702              
  Branches      112      112              
==========================================
- Hits          689      679      -10     
- Misses          3        5       +2     
- Partials       10       18       +8     

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[spider-yamet]

Would appreciate your feedback @Magicbook1108 @yingfeng :)