fix: Unparse random as rand for some dialects

[nuno-faria]

Which issue does this PR close?

• Closes Unparsing the random function generates invalid SQL for some dialects #26008.

Rationale for this change

Generate valid SQL for the dialects that do not support the random() function

What changes are included in this PR?

• Updated the MySQL and BigQuery dialects.

What is the testing strategy for this PR?

Unit test.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.65%. Comparing base (95a1961) to head (db42008).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/sql/src/unparser/dialect.rs	33.33%	2 Missing and 2 partials ⚠️
datafusion/sql/src/unparser/expr.rs	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #26009    +/-   ##
========================================
  Coverage   82.64%   82.65%            
========================================
  Files        1147     1147            
  Lines      445502   446113   +611     
  Branches   445502   446113   +611     
========================================
+ Hits       368179   368720   +541     
- Misses      54969    54999    +30     
- Partials    22354    22394    +40     

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[VVKot]

@nuno-faria maybe link inline to the MySQL/BigQuery docs describing their syntax?

[nuno-faria]

@nuno-faria maybe link inline to the MySQL/BigQuery docs describing their syntax?

Good idea @VVKot, added.