fix: Make relation-generated FK nullable when any relation sharing it asks for nullable

[roxblnfk]

🔍 What was changed

• When several relations generate the same foreign key field (a HasMany and its BelongsTo(nullable: true) on comments.user_id), the column is nullable if any of them asks for nullable — no matter which entity comes first in the registry.
• Fields declared by the user (#[Column], Field in the schema) are not changed by relations, as before.
• Field gets an @internal marker setRelationGenerated() / isRelationGenerated() to tell the two kinds apart.

Review notes

• Only nullability is merged. Other properties of a shared relation-generated field (type, typecast, attributes) are still taken from the first relation, and the morph type column (MorphTrait::ensureMorphField, nullable and length) still works first-come — the same order dependency, left out of scope here.

Checklist

• Fixes Generates nullable columns incorrectly for HasMany relation migrations#17
• How was this tested:
  • Unit tests added (both entity orders; user-declared field kept)
  • Full suite locally on SQLite, PostgreSQL and SQL Server
  • MySQL not run locally
  • Issue reproduction script (annotated entities + migration generator): nullable => true in both orders

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (3a53db9) to head (940d327).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x      #86      +/-   ##
============================================
+ Coverage     95.27%   95.29%   +0.02%     
- Complexity      602      606       +4     
============================================
  Files            46       46              
  Lines          1734     1743       +9     
============================================
+ Hits           1652     1661       +9     
  Misses           82       82              

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