Say how many files Validate checked, including none

[amrali-eg]

Problem

In the GUI, files that pass Validate are not added to the list, and the status was built from the row count. It read "0 files do not have the correct encoding" both when every file passed and when a mistyped mask matched nothing, so a clean result could stand in for a check that never ran. Found in the 2026-09-20 review (F3), from the source.

Change

• A small ValidationTally counts every file the scan reports, from the scan's worker threads (Interlocked), including files that pass and so get no row.
• The Validate status now reads one of:
  • No matching files were examined
  • Checked N files: all valid
  • Checked N files: X do not have the correct encoding, plus , Y could not be read when the scan could not open some files.
• Exclusion coverage (files skipped by name or rule) is still appended as before. View and cancelled statuses are unchanged.
• Backlog entry BL-40.

Tests

ValidationStatusTests feeds real Validate scans into the same tally the window uses:

• a mask that matches nothing, and an empty folder: "No matching files were examined";
• two valid files: "Checked 2 files: all valid";
• valid + windows-1252 + a file held open with FileShare.None: "Checked 3 files: 1 do not have the correct encoding, 1 could not be read";
• valid + windows-1252: the familiar wording with the total.

Mutation checks (each restored byte-for-byte afterwards)

Mutation	Tests failing
No "nothing examined" message	2
Unreadable counted as invalid	1
Valid files not counted as examined	3

Limits

• The two lines that connect the tally to the window (count in the scan callback, describe on completion) are checked by reading, not by a test.
• The GUI smoke suite does not run Validate. Its scan-finished check matches "files processed", which View still writes, so it is unaffected.

Verification

• Based on master after Restore source choices when a GUI review ends without writing #137. Release build, 0 warnings; 975/975 tests pass.
• docs/Test-DefectBacklog.ps1 passes.

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