[Enhancement] Adjust VARCHAR byte length calculation to fully support UTF-8 (utf8mb4)

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/incubator-doris/issues?q=is%3Aissue) and found no similar issues.

### Description

The current logic in `TypeConverter.java` uses a multiplier of `3` to calculate the required byte length for the Doris `VARCHAR` type:

```java
// Current implementation
return length * 3 > 65533
? DorisType.STRING
: String.format("%s(%s)", DorisType.VARCHAR, length * 3);
```

This assumes a maximum of 3 bytes per character, which is insufficient for the widely used utf8mb4 character set (common in MySQL/MariaDB and other sources). The utf8mb4 encoding supports the full range of Unicode characters (including emojis), requiring up to 4 bytes per character.

If a source column contains 4-byte characters, the calculated byte length may underestimate the required size, leading to:

Data truncation or corruption during the synchronization process.

Load failures with errors such as "data length exceeded" or "row size too large" when Doris enforces the byte limit.

### Solution

This change updates the byte multiplier from 3 to 4 to safely accommodate the full utf8mb4 character set, ensuring the calculated byte length is always sufficient for the defined character length, thus guaranteeing data integrity and preventing sync failures.

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start in TypeConverter.java at the VARCHAR byte-length calculation and review how the current multiplier and 65533 limit are applied. Update the calculation to account for up to four UTF-8 bytes per character, then verify that long utf8mb4 values do not produce an undersized VARCHAR definition or exceed the Doris limit.

Written by the indexing model from the issue text.