[Compatibility]: MySQL EXPLAIN FORMAT=JSON is unavailable

### Is there an existing issue for the same bug?

- [x] I have checked the existing issues.

### Branch Name

main

### Commit ID

74668fc075c965c462b30f10ae24a1c19c289cfd

### Other Environment Information

```Markdown
- MatrixOne was built from the commit above and started locally with etc/launch/launch.toml.
- MySQL comparison version: 8.0.45.
- Every statement was repeated three times with the same result.
```

### Actual Behavior

MatrixOne does not provide the MySQL `EXPLAIN FORMAT=JSON` interface.

The MySQL syntax is rejected by the MatrixOne parser:

```sql
EXPLAIN FORMAT=JSON SELECT * FROM target_t WHERE id=1;
```

MatrixOne's parenthesized syntax recognizes `JSON`, but execution rejects it:

```sql
EXPLAIN (FORMAT JSON) SELECT * FROM target_t WHERE id=1;
```

```text
not supported: Unsupport explain format 'json'
```

The same rejection occurs for a table scan, join, CTE, window query, UPDATE,
and DELETE. Ordinary text `EXPLAIN` works for the same statements.

The explain formatter contains explicit JSON NYI branches across query and
node formatting, so this is not limited to one plan node.

### Expected Behavior

MatrixOne should accept MySQL's `EXPLAIN FORMAT=JSON` syntax and return a valid
JSON document describing the plan. MySQL 8.0.45 returns JSON plans for every
statement below.

### Steps to Reproduce

```sql
CREATE DATABASE codex_explain_json_0907;
USE codex_explain_json_0907;

CREATE TABLE target_t(id INT PRIMARY KEY, v INT);
CREATE TABLE source_t(id INT PRIMARY KEY, v INT);
INSERT INTO target_t VALUES(1,10),(2,20),(3,30);
INSERT INTO source_t VALUES(1,101),(2,202),(4,404);

-- MySQL-compatible syntax is rejected by the MatrixOne parser.
EXPLAIN FORMAT=JSON SELECT * FROM target_t WHERE id=1;

-- MatrixOne syntax parses JSON but the formatter rejects it.
EXPLAIN (FORMAT JSON) SELECT * FROM target_t WHERE id=1;
EXPLAIN (FORMAT JSON)
SELECT * FROM target_t t JOIN source_t s ON s.id=t.id;
EXPLAIN (FORMAT JSON)
WITH c AS (SELECT * FROM source_t) SELECT * FROM c WHERE id=1;
EXPLAIN (FORMAT JSON)
SELECT ROW_NUMBER() OVER(ORDER BY id) FROM target_t;
EXPLAIN (FORMAT JSON)
UPDATE target_t SET v=v+1 WHERE id=1;
EXPLAIN (FORMAT JSON)
DELETE FROM target_t WHERE id=1;

-- Control: text format works.
EXPLAIN SELECT * FROM target_t WHERE id=1;
EXPLAIN (FORMAT TEXT) SELECT * FROM target_t WHERE id=1;
```

### Additional information

The JSON formatter paths return explicit NYI errors in
`pkg/sql/plan/explain/explain_node.go` and
`pkg/sql/plan/explain/explain_query.go`.