fix: add relation query composition methods

[3m1n3nc3]

Summary

• Add query composition methods to the shared Relation base.
• Support when, unless, tap, and pipe on relationships.
• Add nested relationship constraints such as whereHas, has, and their OR, absence, and polymorphic variants.
• Support callback and column/value forms of where and orWhere.
• Add OR null predicates.
• Make these capabilities available to MorphManyRelation and all other relation types.

Testing

• 557 tests passed, 2 skipped.
• Lint passed.
• Formatting check passed.
• Package and TypeScript declaration builds passed.

[codecov]

Codecov Report

❌ Patch coverage is 41.79104% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/relationship/Relation.ts	41.79%	39 Missing ⚠️

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