test: achieve 100% coverage on path-utils.ts

Changes:
- Updated Windows path rejection tests to use backslashes as specified
- Added comprehensive pathExists() method tests
- Reordered validation checks in isValidVaultPath() to ensure Windows
  absolute paths are caught before invalid character check
- This fix ensures the Windows drive letter validation is reachable

Coverage improvement: 98.18% -> 100%
Tests added: 3 new test cases
All 512 tests passing

src/utils/path-utils.ts

@@ -59,14 +59,14 @@ export class PathUtils {
 
 		const normalized = this.normalizePath(path);
 
-		// Check for invalid characters (Windows restrictions)
-		const invalidChars = /[<>:"|?*\x00-\x1F]/;
-		if (invalidChars.test(normalized)) {
+		// Check for absolute paths (should be vault-relative)
+		if (normalized.startsWith('/') || /^[A-Za-z]:/.test(normalized)) {
 			return false;
 		}
 
-		// Check for absolute paths (should be vault-relative)
-		if (normalized.startsWith('/') || /^[A-Za-z]:/.test(normalized)) {
+		// Check for invalid characters (Windows restrictions)
+		const invalidChars = /[<>:"|?*\x00-\x1F]/;
+		if (invalidChars.test(normalized)) {
 			return false;
 		}