Slow-query tables do not expose phase-specific backoff types

## Bug Report

`INFORMATION_SCHEMA.SLOW_QUERY` and `CLUSTER_SLOW_QUERY` expose only the legacy `BACKOFF_TYPES` column. Modern slow logs write transaction retry types as `Prewrite_Backoff_types` and `Commit_Backoff_types`, while coprocessor retry types are encoded only in `Cop_backoff__*` detail lines.

As a result, SQL queries and TiDB Dashboard cannot reliably identify which execution phase caused a backoff. For example, a read-only query can contain `txnLockFast` in `BACKOFF_DETAIL` while `BACKOFF_TYPES` is empty.

This is related to #62218, and expands its scope to define a phase-specific, backward-compatible contract for transaction and coprocessor backoff types.

### 1. Minimal reproduce step (Required)

1. Point `tidb_slow_query_file` to a slow-log file containing records such as:

```text
# Time: 2026-09-02T10:00:00+08:00
# Prewrite_Backoff_types: [txnLock]
# Commit_Backoff_types: [regionMiss]
# Cop_backoff_txnLockFast_total_times: 2 Cop_backoff_txnLockFast_total_time: 0.2
select 1;
```

2. Query the record through `INFORMATION_SCHEMA.SLOW_QUERY`:

```sql
SELECT backoff_types, backoff_detail
FROM information_schema.slow_query
WHERE query = 'select 1;';

SELECT prewrite_backoff_types, commit_backoff_types, cop_backoff_types
FROM information_schema.slow_query
WHERE query = 'select 1;';
```

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

The slow-query tables should expose the retry types by phase:

- `PREWRITE_BACKOFF_TYPES = '[txnLock]'`
- `COMMIT_BACKOFF_TYPES = '[regionMiss]'`
- `COP_BACKOFF_TYPES = '[txnLockFast]'`

Historical `Backoff_types` log lines should continue to populate the existing `BACKOFF_TYPES` column without changing its semantics. Cop backoff types should be sorted and deduplicated so the result is stable.

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

- `Prewrite_Backoff_types` and `Commit_Backoff_types` are ignored because the tables do not define matching columns.
- Querying the expected phase-specific columns fails with `Unknown column`.
- Cop backoff types are available only inside the opaque `BACKOFF_DETAIL` text.
- The legacy `BACKOFF_TYPES` column is empty for modern records that no longer contain a `Backoff_types` field.

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

Reproduced on `master` at commit `78cac443a4` on September 2, 2026. The field-name mismatch is also described in #62218, which notes that branches after the slow-log field split are affected.

### Analysis

The slow-log writer and table parser no longer share a complete set of field names. The transaction fields can be mapped directly, while cop backoff types can be derived from existing `Cop_backoff__total_times` lines. Adding separate columns preserves the legacy `BACKOFF_TYPES` behavior and avoids conflating retries from different execution phases.

Start by tracing the INFORMATION_SCHEMA.SLOW_QUERY and CLUSTER_SLOW_QUERY table definitions and the slow-log parser that handles BACKOFF_TYPES and BACKOFF_DETAIL. Verify the supplied sample through the shown SQL queries, then ensure phase-specific transaction and coprocessor columns expose the expected values while legacy BACKOFF_TYPES behavior remains unchanged and cop types are stable and deduplicated.

Written by the indexing model from the issue text.