fix(MySQL): render CURRENT_TIMESTAMP with the column's fractional seconds precision

[roxblnfk]

🔍 What was changed

• datetime / timestamp columns with a size and defaultValue(AbstractColumn::DATETIME_NOW) render DEFAULT CURRENT_TIMESTAMP(N) on MySQL, so datetime(6) no longer fails with error 1067.
• A reflected CURRENT_TIMESTAMP(N) default (MariaDB: current_timestamp(N)) is read as DATETIME_NOW: reading such a column no longer throws, and saving the same declaration again produces no diff.

Review notes

• Changing only the precision of an existing column (datetime → datetime(6)) still produces no ALTER, because size is excluded from MySQL column comparison. That is unchanged here.

Checklist

• Fixes [Bug]: MySQL datetime with precision fails with CURRENT_TIMESTAMP default #277
• How was this tested:
  • Functional tests added (MySQL, live server) and a unit test for the reflected defaults
  • Full suites run locally: MySQL 8.0, PostgreSQL 15, SQL Server 2019, SQLite, Unit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.74%. Comparing base (73a8306) to head (ceda1f0).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #278      +/-   ##
============================================
+ Coverage     95.72%   95.74%   +0.01%     
- Complexity     2246     2251       +5     
============================================
  Files           142      142              
  Lines          6384     6389       +5     
============================================
+ Hits           6111     6117       +6     
+ Misses          273      272       -1     

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