TiDB does not handle transient error raised by PD

## Bug Report

As mentioned in https://github.com/tikv/pd/issues/9326, when PD meets some transient error in network communication, PD does not handle it and raises it to tidb. However, TiDB does not handle it as well, so the SQL execution failed.

This network failure should be handled by either PD or TiDB, but no one resolves it.

### 1. Minimal reproduce step (Required)

drop database and send the following RPC to PD:
```
[2025/05/15 06:43:36.622 +00:00] [INFO] [audit.go:128] ["audit log"] [service-info="{ServiceLabel:PatchRegionLabelRules, Method:HTTP/1.1/PATCH:/pd/api/v1/config/region-label/rules, CallerID:tidb-info-syncer, IP:10.244.1.79, Port:60058, StartTime:2025-05-15 06:43:36 +0000 UTC, URLParam:{}, BodyParam:{\"sets\":[],\"deletes\":[\"schema/access_path_selection/access_path_selection\"]}}"]
```

The return value is service unavailable.
```
[2025/05/15 06:43:36.470 +00:00] [INFO] [session.go:4111] ["CRUCIAL OPERATION"] [conn=3078628316] [schemaVersion=23107] [cur_db=] [sql="drop database access_path_selection"] [user=root@%]
[2025/05/15 06:43:36.552 +00:00] [INFO] [executor.go:6613] ["DDL job submitted"] [category=ddl] [job="ID:22826, Type:drop schema, State:queueing, SchemaState:public, SchemaID:22819, TableID:0, RowCount:0, ArgLen:1, start time: 2025-05-15 06:43:36.444 +0000 UTC, Err:, ErrCount:0, SnapshotVersion:0, Version: v2"] [query="drop database access_path_selection"]
[2025/05/15 06:43:36.570 +00:00] [INFO] [job_submitter.go:139] ["add DDL jobs"] [category=ddl] ["batch count"=1] [jobs="ID:22826, Type:drop schema, State:queueing, SchemaState:public, SchemaID:22819, TableID:0, RowCount:0, ArgLen:1, start time: 2025-05-15 06:43:36.444 +0000 UTC, Err:, ErrCount:0, SnapshotVersion:0, Version: v2; "] [table=true] [fast_create=true]
[2025/05/15 06:43:36.578 +00:00] [INFO] [job_worker.go:784] ["run DDL job"] [category=ddl] [jobID=22826] [conn=3078628316] [job="ID:22826, Type:drop schema, State:queueing, SchemaState:public, SchemaID:22819, TableID:0, RowCount:0, ArgLen:0, start time: 2025-05-15 06:43:36.444 +0000 UTC, Err:, ErrCount:0, SnapshotVersion:0, Version: v2"]
[2025/05/15 06:43:36.656 +00:00] [WARN] [info.go:523] ["response not 200"] [method=PATCH] [host=basic-pd-0.basic-pd-peer.default.svc:2379] [url=/pd/api/v1/config/region-label/rules] ["http status"=500]
[2025/05/15 06:43:36.659 +00:00] [INFO] [job_worker.go:717] ["DDL job is cancelled normally"] [category=ddl] [jobID=22826] [conn=3078628316] [error="[domain:8243]\"rpc error: code = Unavailable desc = outbound unavailable injected by RPCTest\"\n"]
[2025/05/15 06:43:36.661 +00:00] [INFO] [job_worker.go:390] ["finish DDL job"] [category=ddl] [jobID=22826] [conn=3078628316] [job="ID:22826, Type:drop schema, State:cancelled, SchemaState:public, SchemaID:22819, TableID:0, RowCount:0, ArgLen:1, start time: 2025-05-15 06:43:36.444 +0000 UTC, Err:[domain:8243]\"rpc error: code = Unavailable desc = outbound unavailable injected by RPCTest\"\n, ErrCount:1, SnapshotVersion:0, Version: v2"]
[2025/05/15 06:43:36.697 +00:00] [INFO] [executor.go:6738] ["DDL job is failed"] [category=ddl] [jobID=22826]
[2025/05/15 06:43:36.698 +00:00] [INFO] [tidb.go:278] ["rollbackTxn called due to ddl/autocommit failure"]
[2025/05/15 06:43:36.699 +00:00] [WARN] [session.go:2162] ["run statement failed"] [conn=3078628316] [session_alias=] [schemaVersion=23107] [error="[domain:8243]\"rpc error: code = Unavailable desc = outbound unavailable injected by RPCTest\"\n"] [session="{\n \"currDBName\": \"\",\n \"id\": 3078628316,\n \"status\": 2,\n \"strictMode\": true,\n \"user\": {\n \"Username\": \"root\",\n \"Hostname\": \"127.0.0.1\",\n \"CurrentUser\": false,\n \"AuthUsername\": \"root\",\n \"AuthHostname\": \"%\",\n \"AuthPlugin\": \"mysql_native_password\"\n }\n}"]
[2025/05/15 06:43:36.704 +00:00] [INFO] [conn.go:1184] ["command dispatched failed"] [conn=3078628316] [session_alias=] [connInfo="id:3078628316, addr:127.0.0.1:59450 status:10, collation:utf8mb4_general_ci, user:root"] [command=Query] [status="inTxn:0, autocommit:1"] [sql="drop database access_path_selection"] [txn_mode=PESSIMISTIC] [timestamp=458041961072295938] [err="[domain:8243]\"rpc error: code = Unavailable desc = outbound unavailable injected by RPCTest\"\n\ngithub.com/pingcap/errors.AddStack\n\t/root/go/pkg/mod/github.com/pingcap/errors@v0.11.5-0.20240318064555-6bd07397691f/errors.go:178\ngithub.com/pingcap/errors.Trace\n\t/root/go/pkg/mod/github.com/pingcap/errors@v0.11.5-0.20240318064555-6bd07397691f/juju_adaptor.go:15\ngithub.com/pingcap/tidb/pkg/ddl.(*executor).DoDDLJobWrapper\n\t/workspace/source/tidb/pkg/ddl/executor.go:6739\ngithub.com/pingcap/tidb/pkg/ddl.(*executor).doDDLJob2\n\t/workspace/source/tidb/pkg/ddl/executor.go:6555\ngithub.com/pingcap/tidb/pkg/ddl.(*executor).DropSchema\n\t/workspace/source/tidb/pkg/ddl/executor.go:772\ngithub.com/pingcap/tidb/pkg/executor.(*DDLExec).executeDropDatabase\n\t/workspace/source/tidb/pkg/executor/ddl.go:338\ngithub.com/pingcap/tidb/pkg/executor.(*DDLExec).Next\n\t/workspace/source/tidb/pkg/executor/ddl.go:169\ngithub.com/pingcap/tidb/pkg/executor/internal/exec.Next\n\t/workspace/source/tidb/pkg/executor/internal/exec/executor.go:456\ngithub.com/pingcap/tidb/pkg/executor.(*ExecStmt).next\n\t/workspace/source/tidb/pkg/executor/adapter.go:1266\ngithub.com/pingcap/tidb/pkg/executor.(*ExecStmt).handleNoDelayExecutor\n\t/workspace/source/tidb/pkg/executor/adapter.go:1015\ngithub.com/pingcap/tidb/pkg/executor.(*ExecStmt).handleNoDelay\n\t/workspace/source/tidb/pkg/executor/adapter.go:848\ngithub.com/pingcap/tidb/pkg/executor.(*ExecStmt).Exec\n\t/workspace/source/tidb/pkg/executor/adapter.go:611\ngithub.com/pingcap/tidb/pkg/session.runStmt\n\t/workspace/source/tidb/pkg/session/session.go:2288\ngithub.com/pingcap/tidb/pkg/session.(*session).ExecuteStmt\n\t/workspace/source/tidb/pkg/session/session.go:2150\ngithub.com/pingcap/tidb/pkg/server.(*TiDBContext).ExecuteStmt\n\t/workspace/source/tidb/pkg/server/driver_tidb.go:291\ngithub.com/pingcap/tidb/pkg/server.(*clientConn).handleStmt\n\t/workspace/source/tidb/pkg/server/conn.go:2026\ngithub.com/pingcap/tidb/pkg/server.(*clientConn).handleQuery\n\t/workspace/source/tidb/pkg/server/conn.go:1779\ngithub.com/pingcap/tidb/pkg/server.(*clientConn).dispatch\n\t/workspace/source/tidb/pkg/server/conn.go:1378\ngithub.com/pingcap/tidb/pkg/server.(*clientConn).Run\n\t/workspace/source/tidb/pkg/server/conn.go:1147\ngithub.com/pingcap/tidb/pkg/server.(*Server).onConn\n\t/workspace/source/tidb/pkg/server/server.go:741\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_arm64.s:1223"]
```

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

Automatically retry the RPC and handle the transient failure.

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

Neither tidb nor pd handle the failure and raise an error to the user. The SQL execution returns some error and fails.
```
ERROR 8243 (HY000): "rpc error: code = Unavailable desc = transient RPC error"
```

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

We are sure it occurs in v8.5.0. We believe other versions have the same problem.

Start with pkg/ddl/executor.go around lines 6739 and 6555, then follow the drop-database path through pkg/executor/ddl.go:338 to the PD RPC returning code Unavailable. Reproduce the injected transient failure described in the issue; done means the transient RPC is retried or handled and the SQL operation no longer exposes error 8243, with coverage for this failure.

Written by the indexing model from the issue text.