fix(mcp): decode_image_source 非图片输入 OSError 归一 ValueError，守 INVALID_PARAM 契约

评审 fix round 1：Image.open 两处（路径/base64 分支）包 try/except OSError，
合法 base64 或存在文件但非可识别图片时不再让 PIL.UnidentifiedImageError
逃逸出 upload_image 炸 MCP 会话；补 decode 层与工具层覆盖测试各一。

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

tests/test_mcp_tools.py

@@ -101,6 +101,11 @@ class TestDecodeImageSource:
         with pytest.raises(ValueError):
             decode_image_source("!!!not-base64!!!")
 
+    def test_base64_not_image(self):
+        src = base64.b64encode(b"not an image").decode()
+        with pytest.raises(ValueError):
+            decode_image_source(src)
+
 
 class TestTools:
     @pytest.mark.asyncio
@@ -116,6 +121,14 @@ class TestTools:
         assert r["data"]["bytes_bw"] == 15000
         assert r["data"]["slot"] == 0
 
+    @pytest.mark.asyncio
+    async def test_upload_image_not_image(self):
+        src = base64.b64encode(b"not an image").decode()
+        r = await call(make_session_coro(FakeManager()), "upload_image",
+                       {"source": src, "slot": 0})
+        assert r["ok"] is False
+        assert r["error"]["code"] == "INVALID_PARAM"
+
     @pytest.mark.asyncio
     async def test_set_mode_invalid_param(self):
         r = await call(make_session_coro(FakeManager()), "set_mode",