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/dsm_client.py

@@ -61,12 +61,6 @@ def _error_message(code: int, api: str = "") -> str:
         407: "Too many failed login attempts — account temporarily locked",
         408: "IP blocked due to excessive failed attempts",
     }
-    # Docker API codes
-    docker = {
-        1: "Project not found",
-        2: "Container not found",
-    }
-
     if "Auth" in api and code in auth:
         return auth[code]
     if code in common:
@@ -269,7 +263,9 @@ class DsmClient:
         # Log with sensitive fields masked
         log_params = {k: ("***" if k in _SENSITIVE_PARAMS else v) for k, v in req_params.items()}
         retry_tag = " (retry)" if _is_retry else ""
-        logger.debug("DSM GET%s: %s/%s v%d — %s", retry_tag, api, method, resolved_version, log_params)
+        logger.debug(
+            "DSM GET%s: %s/%s v%d — %s", retry_tag, api, method, resolved_version, log_params
+        )
 
         resp = await http.get(url, params=req_params)
         resp.raise_for_status()
@@ -296,6 +292,72 @@ class DsmClient:
 
         raise SynologyError(_error_message(code, api), code=code)
 
+    async def post_request(
+        self,
+        api: str,
+        method: str,
+        version: int | None = None,
+        params: dict[str, Any] | None = None,
+    ) -> dict[str, Any]:
+        """Make a POST (form-encoded) request to the DSM API.
+
+        Identical semantics to request(), but sends params as a form body
+        instead of query-string — required by some Container Manager endpoints
+        (e.g. SYNO.Docker.Image/delete).
+
+        Args:
+            api: DSM API name (e.g. "SYNO.Docker.Image").
+            method: API method (e.g. "delete").
+            version: API version. Defaults to maxVersion from API info.
+            params: Additional form fields.
+
+        Returns:
+            Response data dict from the "data" field of the envelope.
+
+        Raises:
+            SynologyError: On API errors.
+        """
+        sys.stderr.write(f"[dsm] post_request: {api}/{method}\n")
+        sys.stderr.flush()
+        await self._ensure_initialized()
+        http = self._get_http()
+
+        if api not in self._api_cache:
+            raise SynologyError(
+                f"API '{api}' not found. Call query_api_info() first.",
+                code=102,
+            )
+
+        info = self._api_cache[api]
+        resolved_version = version if version is not None else info["maxVersion"]
+        url = f"{self._base_url}/webapi/{info['path']}"
+
+        form: dict[str, Any] = {
+            "api": api,
+            "version": str(resolved_version),
+            "method": method,
+        }
+        if params:
+            form.update(params)
+
+        query_params: dict[str, str] = {}
+        if self._sid:
+            query_params["_sid"] = self._sid
+
+        log_form = {k: ("***" if k in _SENSITIVE_PARAMS else v) for k, v in form.items()}
+        logger.debug("DSM POST: %s/%s v%d — %s", api, method, resolved_version, log_form)
+
+        resp = await http.post(url, params=query_params, data=form)
+        resp.raise_for_status()
+        body = resp.json()
+
+        if body.get("success"):
+            return body.get("data") or {}
+
+        code = body.get("error", {}).get("code", 0)
+        logger.debug("DSM POST response: %s/%s — error code %d", api, method, code)
+        raise SynologyError(_error_message(code, api), code=code)
+
     async def upload_text(
         self,
         dest_folder: str,
@@ -340,7 +402,13 @@ class DsmClient:
         if self._sid:
             query_params["_sid"] = self._sid
 
-        logger.debug("DSM POST: %s/upload v%d — path=%s filename=%s", api, resolved_version, dest_folder, filename)
+        logger.debug(
+            "DSM POST: %s/upload v%d — path=%s filename=%s",
+            api,
+            resolved_version,
+            dest_folder,
+            filename,
+        )
 
         encoded = content.encode("utf-8")
         resp = await http.post(

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", "?")