[KYUUBI #7147] Enforce to use Kyuubi's PlainSASLServer for PLAIN mechanism

[pan3793]

Why are the changes needed?

Close #7147

How was this patch tested?

UT is updated, I also tested it with log4j2 kafka appended (kafka uses SASL/PLAIN authN).

Was this patch authored or co-authored using generative AI tooling?

No.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (a3f1e51) to head (c5c6229).

Files with missing lines	Patch %	Lines
...yuubi/service/authentication/PlainSASLHelper.scala	0.00%	14 Missing ⚠️
...yuubi/service/authentication/PlainSASLServer.scala	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #7145   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files         701     701           
  Lines       43500   43507    +7     
  Branches     5895    5897    +2     
======================================
- Misses      43500   43507    +7     

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