Add wrapper parameter-parity test and backup round-trip test coverage

[ekalb81]

Problem

Test and validation gaps that allow silent drift:

1. Wrapper parity. Windows-PATH-Repair.ps1 duplicates the canonical script's parameter list and forwards via @PSBoundParameters. If Windows-Path-Repair-Full.ps1 gains a parameter, the wrapper silently won't accept or forward it (it already intentionally omits -NoExecute). No test asserts the relationship.
2. Backup functions. Save-PathBackup/Read-PathBackup round-trip is untested, as are Read-PathBackup's rejection paths (missing file, invalid JSON, wrong SchemaVersion/missing properties) and Get-VerifiedDirectoryCandidates. The double-failure path in Invoke-PathUpdate (write fails AND rollback fails → manual-restore message) is also uncovered.
3. Restore validation is too weak. Read-PathBackup (~line 245) validates property presence but not that MachinePath/UserPath are scalar strings. A hand-edited backup whose value is an array or object gets [string]-cast at ~lines 431–432 into garbage like System.Object[], which Test-PathValueSafety will not catch (it only checks length/empty). Malformed values should be rejected at read time.

Proposed fix

• Read-PathBackup: reject backups whose MachinePath/UserPath are not strings (null is acceptable only if explicitly allowed), with a clear error naming the offending property.
• Add Pester tests:
  • Wrapper's parameter set equals the canonical script's minus an explicit allowlist (NoExecute).
  • Round-trip Save-PathBackup → Read-PathBackup under $TestDrive (redirect the backup dir or extract a -BackupDirectory parameter to make it testable).
  • Rejection cases for Read-PathBackup: missing file, invalid JSON, wrong schema version, missing properties, non-string values (array/object/number).
  • Rollback-also-fails case for Invoke-PathUpdate using the injectable scriptblock seams.