feat(models): add type-safe field name references for Pydantic DTOs

[miladmahmoodi]

Add FieldStr utility class and BaseMeta metaclass to BaseDTO so that all DTO subclasses automatically expose field names as type-safe class attributes (e.g., PaginationDTO.page returns FieldStr("page")).

This enables IDE autocompletion, refactoring support, and eliminates hardcoded field name strings. Instance attribute access is unaffected.

• Add FieldStr(str) with slots for memory efficiency
• Add BaseMeta(ModelMetaclass) that sets FieldStr attrs after class construction
• Replace hardcoded field_name strings in range_dtos.py model validators
• Add BDD feature file with 6 scenarios covering FieldStr behavior

Description

Hardcoded field name strings (e.g., "from_", "to") scattered across DTOs and validators are fragile — renaming a field won't cause a lint or type error, leading to silent runtime bugs.

This PR adds a FieldStr(str) utility and a BaseMeta(ModelMetaclass) metaclass to BaseDTO. After Pydantic's ModelMetaclass constructs the class and populates model_fields, BaseMeta overwrites the corresponding class attributes with FieldStr instances so that MyDTO.field_name returns a type-safe string equal to "field_name". Instance attribute access is unaffected because Python resolves instance __dict__ entries before class attributes.

How It Works

class UserDTO(BaseDTO):
    name: str
    email: str

Class-level access returns FieldStr (a str subclass)

UserDTO.name   # FieldStr("name") — type-safe reference
UserDTO.email  # FieldStr("email")

Instance-level access is unaffected

user = UserDTO(name="Alice", email="alice@example.com")
user.name   # "Alice" — actual value

Two hardcoded field name strings in range_dtos.py model validators have been replaced with type-safe FieldStr references as a demonstration.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• 6 new BDD scenarios in features/base_dtos.feature covering: FieldStr class attributes exist, instance access returns actual values, FieldStr works as dict key, FieldStr works in string comparisons, inherited DTOs get FieldStr attributes, existing PaginationDTO has FieldStr attributes
• Ran existing test suite (85 scenarios pass, 4 pre-existing datetime_utils failures unrelated to this change)
• All pre-commit hooks pass (ruff, ruff-format, ty, codespell, trailing-whitespace, end-of-file-fixer, validate-pyproject)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

Additional context

The primary value of this feature is for downstream consumers of ArchiPy who define their own DTOs and can reference field names type-safely in their application code (queries, filters, serialization keys, etc.). Note that @field_validator("field_name") string arguments cannot be replaced because the decorator executes before the class (and thus the metaclass) is fully constructed — this is a Pydantic limitation.