fix(signal): implement send_image_file, send_voice, and send_video for MEDIA: tag delivery

The Signal adapter inherited base class defaults for send_image_file(),
send_voice(), and send_video() which only sent the file path as text
(e.g. '🖼️ Image: /tmp/chart.png') instead of actually delivering the file
as a Signal attachment.

When agent responses contain MEDIA:/path/to/file tags, the gateway
media pipeline extracts them and routes through these methods by file
type. Without proper overrides, image/audio/video files were never
actually delivered to Signal users.

Extract a shared _send_attachment() helper that handles all file
validation, size checking, group/DM routing, and RPC dispatch. The four
public methods (send_document, send_image_file, send_voice, send_video)
now delegate to this helper, following the same pattern used by WhatsApp
(_send_media_to_bridge) and Discord (_send_file_attachment).

The helper also uses a single stat() call with try/except FileNotFoundError
instead of the previous exists() + stat() two-syscall pattern, eliminating
a TOCTOU race. As a bonus, send_document() now gains the 100MB size check
that was previously missing (inconsistency with send_image).

Add 25 tests covering all methods plus MEDIA: tag extraction integration,
method-override guards, and send_document's new size check.

Fixes #5105

gateway/platforms/signal.py

@@ -717,19 +717,27 @@ class SignalAdapter(BasePlatformAdapter):
             return SendResult(success=True)
         return SendResult(success=False, error="RPC send with attachment failed")
 
-    async def send_document(
+    async def _send_attachment(
         self,
         chat_id: str,
         file_path: str,
+        media_label: str,
         caption: Optional[str] = None,
-        filename: Optional[str] = None,
-        **kwargs,
     ) -> SendResult:
-        """Send a document/file attachment."""
+        """Send any file as a Signal attachment via RPC.
+
+        Shared implementation for send_document, send_image_file, send_voice,
+        and send_video — avoids duplicating the validation/routing/RPC logic.
+        """
         await self._stop_typing_indicator(chat_id)
 
-        if not Path(file_path).exists():
-            return SendResult(success=False, error="File not found")
+        try:
+            file_size = Path(file_path).stat().st_size
+        except FileNotFoundError:
+            return SendResult(success=False, error=f"{media_label} file not found: {file_path}")
+
+        if file_size > SIGNAL_MAX_ATTACHMENT_SIZE:
+            return SendResult(success=False, error=f"{media_label} too large ({file_size} bytes)")
 
         params: Dict[str, Any] = {
             "account": self.account,
@@ -746,7 +754,59 @@ class SignalAdapter(BasePlatformAdapter):
         if result is not None:
             self._track_sent_timestamp(result)
             return SendResult(success=True)
-        return SendResult(success=False, error="RPC send document failed")
+        return SendResult(success=False, error=f"RPC send {media_label.lower()} failed")
+
+    async def send_document(
+        self,
+        chat_id: str,
+        file_path: str,
+        caption: Optional[str] = None,
+        filename: Optional[str] = None,
+        **kwargs,
+    ) -> SendResult:
+        """Send a document/file attachment."""
+        return await self._send_attachment(chat_id, file_path, "File", caption)
+
+    async def send_image_file(
+        self,
+        chat_id: str,
+        image_path: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+        **kwargs,
+    ) -> SendResult:
+        """Send a local image file as a native Signal attachment.
+
+        Called by the gateway media delivery flow when MEDIA: tags containing
+        image paths are extracted from agent responses.
+        """
+        return await self._send_attachment(chat_id, image_path, "Image", caption)
+
+    async def send_voice(
+        self,
+        chat_id: str,
+        audio_path: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+        **kwargs,
+    ) -> SendResult:
+        """Send an audio file as a Signal attachment.
+
+        Signal does not distinguish voice messages from file attachments at
+        the API level, so this routes through the same RPC send path.
+        """
+        return await self._send_attachment(chat_id, audio_path, "Audio", caption)
+
+    async def send_video(
+        self,
+        chat_id: str,
+        video_path: str,
+        caption: Optional[str] = None,
+        reply_to: Optional[str] = None,
+        **kwargs,
+    ) -> SendResult:
+        """Send a video file as a Signal attachment."""
+        return await self._send_attachment(chat_id, video_path, "Video", caption)
 
     # ------------------------------------------------------------------
     # Typing Indicators