refactor(endpoints): implement deferred submission and fix clear-all bug

Implement Solution A (complete deferred submission) for custom endpoint
management, replacing the dual-mode system with unified local staging.

Changes:
- Remove immediate backend saves from EndpointSpeedTest
  * handleAddEndpoint: local state update only
  * handleRemoveEndpoint: local state update only
  * handleSelect: remove lastUsed timestamp update
- Add explicit clear detection in ProviderForm
  * Distinguish "user cleared endpoints" from "user didn't modify"
  * Pass empty object {} as clear signal vs null for no-change
- Fix mergeProviderMeta to handle three distinct cases:
  * null/undefined: don't modify endpoints (no meta sent)
  * empty object {}: explicitly clear endpoints (send empty meta)
  * with data: add/update endpoints (overwrite)

Fixed Critical Bug:
When users deleted all custom endpoints, changes were not saved because:
- draftCustomEndpoints=[] resulted in customEndpointsToSave=null
- mergeProviderMeta(meta, null) returned undefined
- Backend interpreted missing meta as "don't modify", preserving old values

Solution:
Detect when user had endpoints and cleared them (hadEndpoints && length===0),
then pass empty object to mergeProviderMeta as explicit clear signal.

Architecture Improvements:
- Transaction atomicity: all fields submitted together on form save
- UX consistency: add/edit modes behave identically
- Cancel button: true rollback with no immediate saves
- Code simplification: removed ~40 lines of immediate save error handling

Testing:
- TypeScript type check: passed
- Rust backend tests: 10/10 passed
- Build: successful

src/utils/providerMetaUtils.ts

@@ -2,9 +2,10 @@ 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 为空对象时，明确删除自定义端点但保留其它元数据。
+ * - 当 customEndpoints 为 null/undefined 时，不修改端点（保留原有端点）。
  * - 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。
+ * - 若结果为空对象且非明确清空场景则返回 undefined，避免写入空 meta。
  */
 export function mergeProviderMeta(
   initialMeta: ProviderMeta | undefined,
@@ -13,6 +14,12 @@ export function mergeProviderMeta(
   const hasCustomEndpoints =
     !!customEndpoints && Object.keys(customEndpoints).length > 0;
 
+  // 明确清空：传入空对象（非 null/undefined）表示用户想要删除所有端点
+  const isExplicitClear =
+    customEndpoints !== null &&
+    customEndpoints !== undefined &&
+    Object.keys(customEndpoints).length === 0;
+
   if (hasCustomEndpoints) {
     return {
       ...(initialMeta ? { ...initialMeta } : {}),
@@ -20,6 +27,25 @@ export function mergeProviderMeta(
     };
   }
 
+  // 明确清空端点
+  if (isExplicitClear) {
+    if (!initialMeta) {
+      // 新供应商且用户没有添加端点（理论上不会到这里）
+      return undefined;
+    }
+
+    if ("custom_endpoints" in initialMeta) {
+      const { custom_endpoints, ...rest } = initialMeta;
+      // 保留其他字段（如 usage_script）
+      // 即使 rest 为空，也要返回空对象（让后端知道要清空 meta）
+      return Object.keys(rest).length > 0 ? rest : {};
+    }
+
+    // initialMeta 中本来就没有 custom_endpoints
+    return { ...initialMeta };
+  }
+
+  // null/undefined：用户没有修改端点，保持不变
   if (!initialMeta) {
     return undefined;
   }