fix: extend caption substring fix to all platforms

Move _merge_caption helper from TelegramAdapter to BasePlatformAdapter
so all adapters inherit it. Fix the same substring-containment bug in:
- gateway/platforms/base.py (photo burst merging)
- gateway/run.py (priority photo follow-up merging)
- gateway/platforms/feishu.py (media batch merging)

The original fix only covered telegram.py. The same bug existed in base.py
and run.py (pure substring check) and feishu.py (list membership without
whitespace normalization).

gateway/platforms/feishu.py

@@ -2065,10 +2065,7 @@ class FeishuAdapter(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.split("\n\n"):
-                existing.text = f"{existing.text}\n\n{event.text}"
+            existing.text = self._merge_caption(existing.text, event.text)
         existing.timestamp = event.timestamp
         if event.message_id:
             existing.message_id = event.message_id