fix(telegram): replace substring caption check with exact line-by-line match

Captions in photo bursts and media group albums were silently dropped when
a shorter caption happened to be a substring of an existing one (e.g.
"Meeting" lost inside "Meeting agenda"). Extract a shared _merge_caption
static helper that splits on "\n\n" and uses exact match with whitespace
normalisation, then use it in both _enqueue_photo_event and
_queue_media_group_event.

Adds 13 unit tests covering the fixed bug scenarios.

Cherry-picked from PR #2671 by Dilee.

gateway/platforms/telegram.py

@@ -2213,6 +2213,22 @@ class TelegramAdapter(BasePlatformAdapter):
             if self._pending_photo_batch_tasks.get(batch_key) is current_task:
                 self._pending_photo_batch_tasks.pop(batch_key, None)
 
+    @staticmethod
+    def _merge_caption(existing_text: Optional[str], new_text: str) -> str:
+        """Merge a new caption into existing text, avoiding duplicates.
+
+        Uses line-by-line exact match (not substring) to prevent false positives
+        where a shorter caption is silently dropped because it appears as a
+        substring of a longer one (e.g. "Meeting" inside "Meeting agenda").
+        Whitespace is normalised for comparison.
+        """
+        if not existing_text:
+            return new_text
+        existing_captions = [c.strip() for c in existing_text.split("\n\n")]
+        if new_text.strip() not in existing_captions:
+            return f"{existing_text}\n\n{new_text}".strip()
+        return existing_text
+
     def _enqueue_photo_event(self, batch_key: str, event: MessageEvent) -> None:
         """Merge photo events into a pending batch and schedule flush."""
         existing = self._pending_photo_batches.get(batch_key)
@@ -2222,10 +2238,7 @@ class TelegramAdapter(BasePlatformAdapter):
             existing.media_urls.extend(event.media_urls)
             existing.media_types.extend(event.media_types)
             if event.text:
-                if not existing.text:
-                    existing.text = event.text
-                elif event.text not in existing.text:
-                    existing.text = f"{existing.text}\n\n{event.text}".strip()
+                existing.text = self._merge_caption(existing.text, event.text)
 
         prior_task = self._pending_photo_batch_tasks.get(batch_key)
         if prior_task and not prior_task.done():
@@ -2415,11 +2428,7 @@ class TelegramAdapter(BasePlatformAdapter):
             existing.media_urls.extend(event.media_urls)
             existing.media_types.extend(event.media_types)
             if event.text:
-                if existing.text:
-                    if event.text not in existing.text.split("\n\n"):
-                        existing.text = f"{existing.text}\n\n{event.text}"
-                else:
-                    existing.text = event.text
+                existing.text = self._merge_caption(existing.text, event.text)
 
         prior_task = self._media_group_tasks.get(media_group_id)
         if prior_task: