修正spacetime-client checkpoint facade的落点

- facade 方法补进真正参与编译的 active/external_generation.rs，此前误写在未被编译的重复文件里
- mapper 重新导出 ExternalGenerationJobProviderCheckpointRecordInput，调用方无需从子模块路径引用
- 删除未被编译的 src/external_generation.rs 重复副本，避免再次改到死文件

server-rs/crates/spacetime-client/src/active/external_generation.rs

@@ -359,6 +359,36 @@ impl SpacetimeClient {
         .await
     }
 
+    /// 写入 provider 任务 checkpoint。
+    ///
+    /// 只在 submit 成功、拿到 provider task id 之后调用一次；procedure 侧对已有
+    /// checkpoint 的 job 直接拒绝，调用方不得用重试来覆盖，这是 at-most-once submit
+    /// 的数据层保证。写入失败意味着本次 attempt 无法安全续跑，按失败处理而不是重新 submit。
+    pub async fn set_external_generation_job_provider_checkpoint(
+        &self,
+        input: ExternalGenerationJobProviderCheckpointRecordInput,
+    ) -> Result<ExternalGenerationJobRecord, SpacetimeClientError> {
+        let procedure_input = input.into();
+
+        self.call_after_connect(
+            "set_external_generation_job_provider_checkpoint_and_return",
+            move |connection, sender| {
+                connection
+                    .procedures()
+                    .set_external_generation_job_provider_checkpoint_and_return_then(
+                        procedure_input,
+                        move |_, result| {
+                            let mapped = result
+                                .map_err(SpacetimeClientError::from_sdk_error)
+                                .and_then(map_external_generation_job_procedure_result);
+                            send_once(&sender, mapped);
+                        },
+                    );
+            },
+        )
+        .await
+    }
+
     pub async fn get_external_generation_job(
         &self,
         input: ExternalGenerationJobGetRecordInput,

server-rs/crates/spacetime-client/src/active/mapper.rs

@@ -85,9 +85,9 @@ pub use self::external_generation::{
     ExternalGenerationJobEnqueueRecordInput, ExternalGenerationJobFailRecordInput,
     ExternalGenerationJobGetRecordInput, ExternalGenerationJobListRecord,
     ExternalGenerationJobListRecordInput, ExternalGenerationJobPhaseUpdateRecordInput,
-    ExternalGenerationJobRecord, ExternalGenerationJobRenewLeaseRecordInput,
-    ExternalGenerationJobSummaryListRecord, ExternalGenerationJobSummaryRecord,
-    ExternalGenerationQueueStatsRecord,
+    ExternalGenerationJobProviderCheckpointRecordInput, ExternalGenerationJobRecord,
+    ExternalGenerationJobRenewLeaseRecordInput, ExternalGenerationJobSummaryListRecord,
+    ExternalGenerationJobSummaryRecord, ExternalGenerationQueueStatsRecord,
 };
 pub use self::llm_router_account::{LlmRouterAccountRecord, LlmRouterAccountUpsertRecordInput};
 pub use self::runtime::FeatureGateConfigRecord;