feat(schema): add common JSON/TOML validators and enforce MCP conditional fields

- Add src/lib/schemas/common.ts with jsonConfigSchema and tomlConfigSchema
- Enhance src/lib/schemas/mcp.ts to require command for stdio and url for http via superRefine
- Keep ProviderForm as-is; future steps will wire new schemas into RHF flows
- Verified: pnpm typecheck passes

src/lib/schemas/provider.ts

@@ -1,19 +1,56 @@
 import { z } from "zod";
 
+/**
+ * 解析 JSON 语法错误，提取位置信息
+ */
+function parseJsonError(error: unknown): string {
+  if (!(error instanceof SyntaxError)) {
+    return "配置 JSON 格式错误";
+  }
+
+  const message = error.message;
+
+  // 提取位置信息：Chrome/V8: "Unexpected token ... in JSON at position 123"
+  const positionMatch = message.match(/at position (\d+)/i);
+  if (positionMatch) {
+    const position = parseInt(positionMatch[1], 10);
+    return `JSON 格式错误：${message.split(" in JSON")[0]}（位置：${position}）`;
+  }
+
+  // Firefox: "JSON.parse: unexpected character at line 1 column 23"
+  const lineColumnMatch = message.match(/line (\d+) column (\d+)/i);
+  if (lineColumnMatch) {
+    const line = lineColumnMatch[1];
+    const column = lineColumnMatch[2];
+    return `JSON 格式错误：第 ${line} 行，第 ${column} 列`;
+  }
+
+  // 通用情况：提取关键错误信息
+  const cleanMessage = message
+    .replace(/^JSON\.parse:\s*/i, "")
+    .replace(/^Unexpected\s+/i, "意外的 ")
+    .replace(/token/gi, "符号")
+    .replace(/Expected/gi, "预期");
+
+  return `JSON 格式错误：${cleanMessage}`;
+}
+
 export const providerSchema = z.object({
   name: z.string().min(1, "请填写供应商名称"),
   websiteUrl: z.string().url("请输入有效的网址").optional().or(z.literal("")),
   settingsConfig: z
     .string()
     .min(1, "请填写配置内容")
-    .refine((value) => {
+    .superRefine((value, ctx) => {
       try {
         JSON.parse(value);
-        return true;
-      } catch {
-        return false;
+      } catch (error) {
+        ctx.addIssue({
+          code: z.ZodIssueCode.custom,
+          message: parseJsonError(error),
+        });
       }
-    }, "配置 JSON 格式错误"),
+    }),
 });
 
 export type ProviderFormData = z.infer<typeof providerSchema>;