[Debug] Add snapshot diff output for CI debugging

Author: krodak

Plugins/BridgeJS/Tests/BridgeJSToolTests/SnapshotTesting.swift

@@ -32,6 +32,23 @@ func assertSnapshot(
             #expect(ok, buildComment(), sourceLocation: sourceLocation)
             if !ok {
                 try input.write(to: URL(fileURLWithPath: actualFilePath))
+                // Print diff for CI debugging
+                let expectedLines = existingSnapshot.split(separator: "\n", omittingEmptySubsequences: false)
+                let actualStr = String(data: input, encoding: .utf8)!
+                let actualLines = actualStr.split(separator: "\n", omittingEmptySubsequences: false)
+                let maxLines = max(expectedLines.count, actualLines.count)
+                for i in 0..<maxLines {
+                    let exp = i < expectedLines.count ? String(expectedLines[i]) : "<missing>"
+                    let act = i < actualLines.count ? String(actualLines[i]) : "<missing>"
+                    if exp != act {
+                        print("SNAPSHOT DIFF line \(i + 1):")
+                        print("  expected: \(exp)")
+                        print("  actual:   \(act)")
+                    }
+                }
+                if expectedLines.count != actualLines.count {
+                    print("SNAPSHOT DIFF line count: expected=\(expectedLines.count) actual=\(actualLines.count)")
+                }
             }
         } else {
             try input.write(to: snapshotPath)