Fix Python 3.14 dict iteration error in JsonModel

[abrookins]

Summary

Fixes #763 - Python 3.14 compatibility for JsonModel.schema_for_fields().

The Problem

In Python 3.14, iterating over cls.__dict__.items() directly can raise:

RuntimeError: dictionary changed size during iteration

This occurs during class construction when Pydantic is still modifying the class namespace.

The Solution

Instead of copying the entire __dict__ before iteration (which wastes memory), this PR iterates over cls.__annotations__ keys and looks up each value in __dict__ individually:

# Before (original - crashes on 3.14):
for name, field in cls.__dict__.items():
    if isinstance(field, FieldInfo):
        ...

# After (this PR):
for name in cls.__annotations__:
    field = cls.__dict__.get(name)
    if isinstance(field, FieldInfo):
        ...

This approach:

• ✅ Avoids iterating over a mutating dict
• ✅ No memory overhead from copying __dict__
• ✅ More targeted - only checks annotated fields (which is what we care about)
• ✅ Works on all Python versions

Tests Added

Comprehensive test coverage for schema_for_fields() edge cases:

• Indexed fields (text, numeric, tag, sortable, fulltext)
• Optional fields with defaults
• Inherited fields from parent models
• Embedded models
• List[str] fields
• FieldInfo annotation verification
• Custom primary keys
• Case-sensitive fields

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@abrookins