Fix Swift Testing error matcher failing on Linux (#1657)

Cast the thrown error to SpecParsingError instead of coercing it to
CustomStringConvertible, which goes through NSError bridging and yields a
different description on Linux.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: yonaskolb

Tests/ProjectSpecTests/InvalidConfigsFormatTests.swift

@@ -36,7 +36,7 @@ struct InvalidConfigsMappingFormatTests {
         #expect {
             try Project(path: arguments.fixturePath)
         } throws: { actualError in
-            (actualError as any CustomStringConvertible).description
+            (actualError as? SpecParsingError)?.description
             == arguments.expectedError.description
         }
     }