Fix Sentry 31NG/368D/31G1: skip Gutenberg media-URL replacement when remote mediaId is null (#22871)

BlockProcessor reads MediaFile.mediaId as a non-null Kotlin String at the
field initializer, but MediaFile is a Java helper whose mediaId is null
whenever the underlying MediaModel has no remote ID (set explicitly in
FluxCUtils.mediaFileFromMediaModel). Guard the single entry point that
constructs MediaUploadCompletionProcessor so the substitution is skipped
instead of crashing: with no remote ID there is nothing to substitute.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nbradbury

WordPress/src/main/java/org/wordpress/android/ui/posts/PostUtils.java

@@ -428,7 +428,10 @@ public static boolean shouldShowGutenbergEditor(boolean isNewPost, String postCo
     public static String replaceMediaFileWithUrlInGutenbergPost(@NonNull String postContent,
                                                                 @NonNull String localMediaId, MediaFile mediaFile,
                                                                 @NonNull String siteUrl) {
-        if (mediaFile != null && contentContainsGutenbergBlocks(postContent)) {
+        // BlockProcessor reads MediaFile.mediaId as a non-null Kotlin String and would NPE;
+        // with no remote ID there is nothing to substitute, so leave the content untouched.
+        if (mediaFile != null && mediaFile.getMediaId() != null
+                && contentContainsGutenbergBlocks(postContent)) {
             MediaUploadCompletionProcessor processor = new MediaUploadCompletionProcessor(localMediaId, mediaFile,
                     siteUrl);
             postContent = processor.processContent(postContent);

WordPress/src/test/java/org/wordpress/android/ui/posts/PostUtilsUploadProcessingTest.kt

@@ -61,4 +61,14 @@ class PostUtilsUploadProcessingTest {
         )
         Assertions.assertThat(processedContent).isEqualTo(TestContent.newMediaTextBlockWithVideo)
     }
+
+    @Test
+    fun `replaceMediaFileWithUrlInGutenbergPost leaves content unchanged when mediaId is null`() {
+        whenever(mediaFile.mediaId).thenReturn(null)
+        val processedContent = PostUtils.replaceMediaFileWithUrlInGutenbergPost(
+            TestContent.oldImageBlock,
+            TestContent.localMediaId, mediaFile, TestContent.siteUrl
+        )
+        Assertions.assertThat(processedContent).isEqualTo(TestContent.oldImageBlock)
+    }
 }