Fix #21317: Replace colons with underscores in window filenames for Windows compatibility

The default FilenamePolicy uses Instant.toString() for Beam Windows, which
produces ISO-8601 format (e.g., 2021-12-31T23:00:00.000Z) containing colons.
Colons are illegal characters in Windows file paths, causing
InvalidPathException when using TextIO.write().withWindowedWrites() on Windows.

This fix replaces colons with underscores ONLY on Windows to ensure
cross-platform compatibility without breaking existing filenames on other OSes.

Fixes #21317

Author: liferoad

sdks/java/core/src/main/java/org/apache/beam/sdk/io/DefaultFilenamePolicy.java

@@ -353,7 +353,15 @@ private String windowToString(BoundedWindow window) {
     }
     if (window instanceof IntervalWindow) {
       IntervalWindow iw = (IntervalWindow) window;
-      return String.format("%s-%s", iw.start().toString(), iw.end().toString());
+      // Use ISO-8601 format but replace colons with underscores for Windows compatibility
+      // since colons are illegal characters in Windows file paths.
+      String startStr = iw.start().toString();
+      String endStr = iw.end().toString();
+      if (System.getProperty("os.name").startsWith("Windows")) {
+        startStr = startStr.replace(':', '_');
+        endStr = endStr.replace(':', '_');
+      }
+      return String.format("%s-%s", startStr, endStr);
     }
     return window.toString();
   }