fix: validate that all characters after first padding character in Base64 decode

[priyanshuvishwakarma273403]

Description

Fixes a bug where Base64.decode accepted malformed inputs with data characters appearing after a padding character = (e.g. "QQ=Q").

Related Issue

Fixes #7482

Changes

• Updated Base64.decode to assert that all characters starting from the first = character to the end of the input string are also =.
• Added unit test cases in Base64Test.java to prevent regression.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.88%. Comparing base (a508fd2) to head (770e092).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7491   +/-   ##
=========================================
  Coverage     79.88%   79.88%           
- Complexity     7340     7343    +3     
=========================================
  Files           809      809           
  Lines         23847    23851    +4     
  Branches       4694     4697    +3     
=========================================
+ Hits          19050    19054    +4     
- Misses         4035     4036    +1     
+ Partials        762      761    -1     

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[DenizAltunkapan]

@priyanshuvishwakarma273403 very good, thank you.