fix(providers): preserve custom endpoints in meta during add/edit operations

Fixed two critical data loss bugs where user-added custom endpoints were discarded:

1. **AddProviderDialog**: Form submission ignored values.meta from ProviderForm and
   re-inferred URLs only from presets/config, causing loss of endpoints added via
   speed test modal. Now prioritizes form-collected meta and uses fallback inference
   only when custom_endpoints is missing.

2. **ProviderForm**: Edit mode always returned initialData.meta, discarding any
   changes made in the speed test modal. Now uses mergeProviderMeta to properly
   merge customEndpointsMap with existing meta fields.

Changes:
- Extract mergeProviderMeta utility to handle meta field merging logic
- Preserve other meta fields (e.g., usage_script) during endpoint updates
- Unify new/edit code paths to use consistent meta handling
- Add comprehensive unit tests for meta merging scenarios
- Add integration tests for AddProviderDialog submission flow

Impact:
- Third-party and custom providers can now reliably manage multiple endpoints
- Edit operations correctly reflect user modifications
- No data loss for existing meta fields like usage_script

src/utils/providerMetaUtils.ts

@@ -0,0 +1,36 @@
+import type { CustomEndpoint, ProviderMeta } from "@/types";
+
+/**
+ * 合并供应商元数据中的自定义端点。
+ * - 当 customEndpoints 为空对象或 null 时，移除自定义端点但保留其它元数据。
+ * - 当 customEndpoints 存在时，覆盖原有自定义端点。
+ * - 若结果为空对象则返回 undefined，避免写入空 meta。
+ */
+export function mergeProviderMeta(
+  initialMeta: ProviderMeta | undefined,
+  customEndpoints:
+    | Record<string, CustomEndpoint>
+    | null
+    | undefined,
+): ProviderMeta | undefined {
+  const hasCustomEndpoints =
+    !!customEndpoints && Object.keys(customEndpoints).length > 0;
+
+  if (hasCustomEndpoints) {
+    return {
+      ...(initialMeta ? { ...initialMeta } : {}),
+      custom_endpoints: customEndpoints!,
+    };
+  }
+
+  if (!initialMeta) {
+    return undefined;
+  }
+
+  if ("custom_endpoints" in initialMeta) {
+    const { custom_endpoints, ...rest } = initialMeta;
+    return Object.keys(rest).length > 0 ? rest : undefined;
+  }
+
+  return { ...initialMeta };
+}