fix(discord): chunk long editMessage payloads and guard progress null race

- editMessage now splits >2000-char text: edits the tracked message with the
  first chunk and appends the rest as follow-up messages, instead of throwing
  Discord "Invalid Form Body … Must be 2000 or fewer" when promoting a long
  owner result over a progress message.
- syncTrackedProgressMessage captures latestProgressText in a local before the
  editMessage await, fixing a race where a concurrent resetProgressState()
  nulled it and crashed with "null is not an object (this.latestProgressText.length)".
- Add editMessage regression tests (in-place edit vs. chunked overflow).

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

src/message-turn-controller.ts

@@ -596,15 +596,19 @@ export class MessageTurnController {
       return;
     }
 
+    // Capture the value before the await: a concurrent resetProgressState()
+    // can null `latestProgressText` while editMessage is in flight, which used
+    // to throw "null is not an object (evaluating 'this.latestProgressText.length')".
+    const progressText = this.latestProgressText;
     if (
       !this.progressMessageId ||
       !this.options.channel.editMessage ||
-      !this.latestProgressText
+      !progressText
     ) {
       return;
     }
 
-    const rendered = this.renderProgressMessage(this.latestProgressText);
+    const rendered = this.renderProgressMessage(progressText);
 
     try {
       await this.options.channel.editMessage(
@@ -616,7 +620,7 @@ export class MessageTurnController {
       this.progressEditFailCount = 0;
       this.logOutboundAudit('progress-edit', {
         messageId: this.progressMessageId,
-        textLength: this.latestProgressText.length,
+        textLength: progressText.length,
         renderedLength: rendered.length,
       });
     } catch (err) {