BR: checksum-pitr loadPitrIdMap swallows mysql.tidb_pitr_id_map query failures (checked error slot not used; silent skip with empty filter)

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

1. Prepare a TiDB cluster and a BR binary built from any version since #64383 (2025-12).
2. Make the query to `mysql.tidb_pitr_id_map` fail — e.g. revoke the privilege needed for the restricted SQL on that table, or make PD/TiKV unavailable during the check.
3. Run `br pitr checksum table` with a filter matching no table (empty-result path).

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

The command fails with an error pointing at the real query failure.

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

The failure is silently swallowed. `loadPitrIdMap` (br/pkg/task/operator/checksum_table.go) checks the fresh query error `errSQL` but returns `errors.Annotate(err, ...)` where `err` is a stale `CreateSession` error slot — provably nil at that point (the `if err != nil` return above already consumed it). `errors.Annotate(nil, msg)` returns nil, so a failed query returns `(nil, nil)`:

- with a non-empty filter: a misleading `no db map found by db name` error later (blames missing id-map data instead of the real failure);
- with an empty filter: `RunPitrChecksumTable` sees err==nil, the request list stays empty, `runChecksum` succeeds — **the PITR data-integrity check is skipped entirely with exit 0**.

Root cause:

```go
rows, _, errSQL := execCtx.ExecRestrictedSQL(...)
if errSQL != nil {
return nil, errors.Annotate(err, "failed to get pitr id map from mysql.tidb_pitr_id_map") // wraps stale `err` (nil)
}
```

Production reachability: medium — trigger is any failure of the `mysql.tidb_pitr_id_map` query during checksum-pitr (permissions drift, PD/TiKV unavailability, upgrade windows); the silent-skip branch additionally needs the filter to match no table. Consequence: silent skip of the PITR data-integrity check with exit 0.

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

```console
Code-verified on pinned ca95cc55e1b and master 16c97eb67f9 (identical code); introduced by pingcap/tidb#64383 (2025-12-03); no upstream fix.
```

Start in br/pkg/task/operator/checksum_table.go at loadPitrIdMap and trace the empty-filter path through RunPitrChecksumTable. Reproduce a failing mysql.tidb_pitr_id_map query with br pitr checksum table; done means the real query failure is reported and the check cannot silently succeed with an empty request list.

Written by the indexing model from the issue text.