fix: tighten telegram docker-media salvage follow-ups

Follow-up on top of the helix4u #6392 cherry-pick:
- reuse one helper for actionable Docker-local file-not-found errors
  across document/image/video/audio local-media send paths
- include /outputs/... alongside /output/... in the container-local
  path hint
- soften the gateway startup warning so it does not imply custom
  host-visible mounts are broken; the warning now targets the specific
  risky pattern of emitting container-local MEDIA paths without an
  explicit export mount
- add focused regressions for /outputs/... and non-document media hint
  coverage

This keeps the salvage aligned with the actual MEDIA delivery problem on
current main while reducing false-positive operator messaging.

gateway/platforms/telegram.py

@@ -1657,6 +1657,21 @@ class TelegramAdapter(BasePlatformAdapter):
         except Exception as exc:
             logger.error("Failed to write update response from callback: %s", exc)
 
+    def _missing_media_path_error(self, label: str, path: str) -> str:
+        """Build an actionable file-not-found error for gateway MEDIA delivery.
+
+        Paths like /workspace/... or /output/... often only exist inside the
+        Docker sandbox, while the gateway process runs on the host.
+        """
+        error = f"{label} file not found: {path}"
+        if path.startswith(("/workspace/", "/output/", "/outputs/")):
+            error += (
+                " (path may only exist inside the Docker sandbox. "
+                "Bind-mount a host directory and emit the host-visible "
+                "path in MEDIA: for gateway file delivery.)"
+            )
+        return error
+
     async def send_voice(
         self,
         chat_id: str,
@@ -1673,7 +1688,7 @@ class TelegramAdapter(BasePlatformAdapter):
         try:
             import os
             if not os.path.exists(audio_path):
-                return SendResult(success=False, error=f"Audio file not found: {audio_path}")
+                return SendResult(success=False, error=self._missing_media_path_error("Audio", audio_path))
             
             with open(audio_path, "rb") as audio_file:
                 # .ogg files -> send as voice (round playable bubble)
@@ -1722,7 +1737,7 @@ class TelegramAdapter(BasePlatformAdapter):
         try:
             import os
             if not os.path.exists(image_path):
-                return SendResult(success=False, error=f"Image file not found: {image_path}")
+                return SendResult(success=False, error=self._missing_media_path_error("Image", image_path))
 
             _thread = self._metadata_thread_id(metadata)
             with open(image_path, "rb") as image_file:
@@ -1759,14 +1774,7 @@ class TelegramAdapter(BasePlatformAdapter):
 
         try:
             if not os.path.exists(file_path):
-                error = f"File not found: {file_path}"
-                if file_path.startswith(("/workspace/", "/output/")):
-                    error += (
-                        " (path may only exist inside the Docker sandbox. "
-                        "Bind-mount a host directory and emit the host-visible "
-                        "path in MEDIA: for gateway file delivery.)"
-                    )
-                return SendResult(success=False, error=error)
+                return SendResult(success=False, error=self._missing_media_path_error("File", file_path))
 
             display_name = file_name or os.path.basename(file_path)
             _thread = self._metadata_thread_id(metadata)
@@ -1800,7 +1808,7 @@ class TelegramAdapter(BasePlatformAdapter):
 
         try:
             if not os.path.exists(video_path):
-                return SendResult(success=False, error=f"Video file not found: {video_path}")
+                return SendResult(success=False, error=self._missing_media_path_error("Video", video_path))
 
             _thread = self._metadata_thread_id(metadata)
             with open(video_path, "rb") as f: