Fix delete_image: POST with images JSON array (DevTools-confirmed format)

DSM Container Manager rejects name+tag and sha256 id params (error 114).
Browser DevTools capture shows the correct call is:

  POST SYNO.Docker.Image / delete / version=1
  images=[{"repository":"nouchka/sqlite3","tags":["latest"]}]

Changes:
- Add DsmClient.post_request() for form-encoded POST requests
- delete_image now calls post_request with version=1 and the images
  JSON array built from the resolved repository name and tag
- Remove unused docker error-code dict from _error_message()
- Tests mock post_request and assert the images param content

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

src/mcp_synology_container/modules/images.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 import logging
 import sys
 from datetime import UTC, datetime
@@ -243,17 +244,22 @@ def register_images(mcp: FastMCP, config: AppConfig, client: DsmClient) -> None:
                 f"Call delete_image(image_id={image_id!r}, confirmed=True) to confirm."
             )
 
-        # 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")
+        # DSM Container Manager expects a POST with version=1 and an
+        # "images" JSON array — confirmed via browser DevTools capture.
+        # Format: images=[{"repository": "nginx", "tags": ["1.24"]}]
+        delete_repo = repo
+        delete_tag = img_tags[0] if img_tags else tag
+        images_param = json.dumps([{"repository": delete_repo, "tags": [delete_tag]}])
+        sys.stderr.write(
+            f"[delete_image] POST SYNO.Docker.Image/delete v1 images={images_param!r}\n"
+        )
         sys.stderr.flush()
         try:
-            await client.request(
+            await client.post_request(
                 "SYNO.Docker.Image",
                 "delete",
-                params={"id": img_hash},
+                version=1,
+                params={"images": images_param},
             )
         except Exception as e:
             code = getattr(e, "code", "?")