fix(msteams): prevent duplicate text when stream exceeds 4000 char limit (#59297)

When a streamed response exceeds TEAMS_MAX_CHARS, the stream sets streamFailed=true and finalizes. Previously, hasContent returned false when streamFailed was true, causing preparePayload to pass through the full payload for block delivery, duplicating already-streamed text.

Now tracks streamed length and strips the already-delivered prefix from fallback payloads.

Fixes #58601

thanks @bradgroux

extensions/msteams/src/reply-stream-controller.ts

@@ -55,17 +55,44 @@ export function createTeamsReplyStreamController(params: {
     },
 
     preparePayload(payload: ReplyPayload): ReplyPayload | undefined {
-      if (!stream || !streamReceivedTokens || !stream.hasContent || stream.isFinalized) {
+      if (!stream || !streamReceivedTokens) {
         return payload;
       }
 
-      // Stream handled this text segment — finalize it and reset so any
+      const hasMedia = Boolean(payload.mediaUrl || payload.mediaUrls?.length);
+
+      // Stream failed after partial delivery (e.g. > 4000 chars). Send only
+      // the unstreamed suffix via block delivery to avoid duplicate text.
+      if (stream.isFailed) {
+        streamReceivedTokens = false;
+
+        if (!payload.text) {
+          return payload;
+        }
+
+        const streamedLength = stream.streamedLength;
+        if (streamedLength <= 0) {
+          return payload;
+        }
+
+        const remainingText = payload.text.slice(streamedLength);
+        if (!remainingText) {
+          return hasMedia ? { ...payload, text: undefined } : undefined;
+        }
+
+        return { ...payload, text: remainingText };
+      }
+
+      if (!stream.hasContent || stream.isFinalized) {
+        return payload;
+      }
+
+      // Stream handled this text segment. Finalize it and reset so any
       // subsequent text segments (after tool calls) use fallback delivery.
       // finalize() is idempotent; the later call in markDispatchIdle is a no-op.
       streamReceivedTokens = false;
       pendingFinalize = stream.finalize();
 
-      const hasMedia = Boolean(payload.mediaUrl || payload.mediaUrls?.length);
       if (!hasMedia) {
         return undefined;
       }