Fix delete_image: use sha256 image ID instead of name+tag

DSM SYNO.Docker.Image/delete returns error 114 when called with name+tag.
The API expects the sha256 hash from the image list (field "id") as the
"id" parameter.

- Look up the sha256 hash from SYNO.Docker.Image/list (already fetched
  for the in-use check), then pass params={"id": img_hash}
- Guard against missing hash with a clear error message
- Updated tests to assert id param is sent, name/tag are absent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mcp_synology_container/modules/images.py

@@ -243,20 +243,17 @@ def register_images(mcp: FastMCP, config: AppConfig, client: DsmClient) -> None:
                 f"Call delete_image(image_id={image_id!r}, confirmed=True) to confirm."
             )
 
-        # Perform deletion using the resolved name+tag.
-        # N4S4 reference: SYNO.Docker.Image / delete, params: name, tag
-        delete_name = repo
-        delete_tag = img_tags[0] if img_tags else tag
-        sys.stderr.write(
-            f"[delete_image] api=SYNO.Docker.Image method=delete"
-            f" name={delete_name!r} tag={delete_tag!r}\n"
-        )
+        # DSM requires the sha256 image ID for deletion, not name+tag.
+        if not img_hash:
+            return f"Cannot delete '{display_name}': image ID (sha256) not found in list."
+
+        sys.stderr.write(f"[delete_image] api=SYNO.Docker.Image method=delete id={img_hash!r}\n")
         sys.stderr.flush()
         try:
             await client.request(
                 "SYNO.Docker.Image",
                 "delete",
-                params={"name": delete_name, "tag": delete_tag},
+                params={"id": img_hash},
             )
         except Exception as e:
             code = getattr(e, "code", "?")