[resource-control] Cancelled ALTER RESOURCE GROUP leaves the uncommitted configuration active in PD

## Bug Report

> Evidence status: confirmed.

### 1. Minimal reproduce step (Required)

```sql
Create resource group ai_rg_real at RU_PER_SEC=1000 PRIORITY=LOW. Pause the DDL worker immediately after infosync.ModifyResourceGroup changes PD during ALTER to RU_PER_SEC=1 PRIORITY=HIGH. From another session run ADMIN CANCEL DDL JOBS. Release the worker and compare the DDL result, SHOW CREATE RESOURCE GROUP, INFORMATION_SCHEMA.RESOURCE_GROUPS, and DDL job history.
```

### 2. What did you expect to see? (Required)

A cancelled DDL must leave both metadata and runtime resource-manager state at 1000/LOW.

### 3. What did you see instead? (Required)

The ALTER returns ERROR 8214 and job history is cancelled; SHOW CREATE is 1000/LOW, while the PD-backed runtime view is 1/HIGH.

### 4. What is your TiDB version? (Required)

master 13282a8bd06b; non-partition ALTER RESOURCE GROUP

Likely root cause and fix direction

onAlterResourceGroup stages metadata in the worker transaction but commits the external PD ModifyResourceGroup before schema/job publication. Cancellation causes a job-row write conflict and generic rollback only marks the job cancelled; it has no PD compensation.