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 in the window string format to
ensure cross-platform compatibility.

Author: liferoad

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

@@ -353,7 +353,11 @@ 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().replace(':', '_');
+      String endStr = iw.end().toString().replace(':', '_');
+      return String.format("%s-%s", startStr, endStr);
     }
     return window.toString();
   }