[fix](planner) : Incorrect result of CAST AS CHAR

[lsy3993]

Proposed changes

Issue Number: #3861

Problem summary

when cast string to char(int), fe should judge the length of target char, rather than return whole string

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[BiteTheDDDDt]

We are not doing char type length processing in the execution engine, it seems that this will make us not have consistent performance globally

[lsy3993]

This bug fix is only for the case of casting constant string in FE. When the cast statement calculates in BE, we may fix it later

[zy-kkk]

LGTM

[github-actions]

PR approved by anyone and no changes requested.