feat(filters): add RelationshipFilter for relationship-based queries

[cofin]

Summary

Adds RelationshipFilter class enabling efficient filtering across SQLAlchemy relationships using EXISTS subqueries. This addresses:

• Issue Enhancement: Add StatementFilters for relationships & association object patterns #364: Add StatementFilters for relationships & association object patterns
• Issue MultiFilter with Many-to-Many relationships #505: Fix MultiFilter with Many-to-Many generating incorrect SQL

Features

• Support for one-to-many, many-to-one, many-to-many relationships
• Nested relationship filtering (3+ levels)
• negate parameter for NOT EXISTS queries
• CollectionFilter auto-detection of relationship attributes
• MultiFilter JSON API integration

Example Usage

# Filter orders by customer country (one-to-many)
RelationshipFilter(
    relationship="customer",
    filters=[CollectionFilter("country", ["USA", "Canada"])]
)

# Filter users by tags (many-to-many)
RelationshipFilter(
    relationship="tags",
    filters=[CollectionFilter("name", ["admin", "moderator"])]
)

# Nested filtering (Order -> Customer -> Tier)
RelationshipFilter(
    relationship="customer",
    filters=[
        RelationshipFilter(
            relationship="tier",
            filters=[ComparisonFilter("level", "ge", 3)]
        )
    ]
)

Closes #364
Closes #505

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.52756% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.09%. Comparing base (8916358) to head (cc4261a).

Files with missing lines	Patch %	Lines
advanced_alchemy/filters.py	79.52%	16 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #639      +/-   ##
==========================================
- Coverage   79.09%   79.09%   -0.01%     
==========================================
  Files          99       99              
  Lines        7861     7964     +103     
  Branches     1065     1086      +21     
==========================================
+ Hits         6218     6299      +81     
- Misses       1301     1317      +16     
- Partials      342      348       +6     

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