[Bug]: utf8mb4_0900_ai_ci string comparison and REGEXP are case-sensitive

### Environment

- Branch: latest `main`
- Commit: `33e96877604ad4b847ea7d5c55eb09429de1a5b6`
- MySQL oracle: 8.0.45
- MO: `8.0.30-MatrixOne-v1.3.0`

### Steps to Reproduce

```sql
set names utf8mb4 collate utf8mb4_0900_ai_ci;
select @@character_set_connection as cs, @@collation_connection as coll;
select 'a' = 'A' as eq_case,
'a ' = 'a' as eq_trailing,
'abc' regexp 'A' as regexp_case;
```

### MySQL Result

```text
cs coll
utf8mb4 utf8mb4_0900_ai_ci

eq_case eq_trailing regexp_case
1 0 1
```

### MatrixOne Result

```text
cs coll
utf8mb4 default

eq_case eq_trailing regexp_case
0 0 0
```

### Expected Behavior

For a case-insensitive collation such as `utf8mb4_0900_ai_ci`, string equality and REGEXP matching should follow MySQL's case-insensitive comparison semantics. The trailing-space equality result `0` is already aligned for this collation.