fix: recursively convert nested dicts in model_from_dict

[cofin]

Summary

Fixes #556 - Regression where service.create() failed when passing nested dictionaries for relationship data.

Changes

• Add _convert_relationship_value() helper function to handle relationship conversion
• Modify model_from_dict() to detect RelationshipProperty attributes using SQLAlchemy's class_mapper
• Recursively convert nested dicts for one-to-one and many-to-one relationships
• Convert list of dicts for one-to-many and many-to-many relationships
• Handle None, empty lists, and mixed lists (dicts + model instances) correctly
• Preserve backward compatibility for non-nested usage

Example Usage

data = {
    "name": "John Doe",
    "profile": {"bio": "Developer"},  # Converted to Profile instance
    "addresses": [
        {"street": "123 Main St"},
        {"street": "456 Oak Ave"}
    ]  # Each dict converted to Address instance
}
user = model_from_dict(User, **data)

Tests Added

• 17 comprehensive unit tests covering:
  • Single nested dict (one-to-one)
  • List of nested dicts (one-to-many)
  • Deeply nested structures (3+ levels)
  • None and empty list handling
  • Mixed lists (dicts + instances)
  • Many-to-many relationships
  • Performance benchmarks

Test plan

• All 184 unit tests pass
• Type checking passes (mypy + pyright)
• Linting passes (ruff check + format)
• Pre-commit hooks pass
• No manual edits to _sync.py files

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.49%. Comparing base (2bf5157) to head (5cd8cec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
+ Coverage   80.43%   80.49%   +0.06%     
==========================================
  Files          87       87              
  Lines        6451     6471      +20     
  Branches      838      845       +7     
==========================================
+ Hits         5189     5209      +20     
  Misses        998      998              
  Partials      264      264              

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[JacobCoffee]

couldnt you just do attr.uselis since itd always be a bool

Suggested change

	is_collection=attr.uselist or False,
	is_collection=attr.uselist,