The returned datatype of case when is inconsistent with mysql

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
select case false when true then 1.19999999999 else 1 end ;
```
### 2. What did you expect to see? (Required)
results from mysql
```
MySQL [fuzztest]> select case false when true then 1.19999999999 else 1 end ;
+----------------------------------------------------+
| case false when true then 1.19999999999 else 1 end |
+----------------------------------------------------+
| 1 |
+----------------------------------------------------+
1 row in set (0.00 sec)
```

### 3. What did you see instead (Required)
results from tidb
```
MySQL [test1]> select case false when true then 1.19999999999 else 1 end ;
+----------------------------------------------------+
| case false when true then 1.19999999999 else 1 end |
+----------------------------------------------------+
| 1.00000000000 |
+----------------------------------------------------+
1 row in set (0.01 sec)
```

### 4. What is your TiDB version? (Required)
```
MySQL [test1]> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v8.5.1
Edition: Community
Git Commit Hash: fea86c8e35ad4a86a5e1160701f99493c2ee547c
Git Branch: HEAD
UTC Build Time: 2025-01-16 07:38:34
GoVersion: go1.23.4
Race Enabled: false
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

Start by running the reported CASE query in TiDB and MySQL and compare the result types and displayed values. Trace the CASE expression's type inference and numeric result handling; done means TiDB matches MySQL for this reproduction without regressing related CASE expressions.

Written by the indexing model from the issue text.