fix: migrate GeometryValidatorMixin to Pydantic V2 field_validator

The V1-style @validator decorator is deprecated in Pydantic 2.x and
scheduled for removal in V3. Migrates GeometryValidatorMixin's geometry
validator to the V2 @field_validator API, adding the explicit
@classmethod that V2 requires (V1 handled this implicitly).

Validator logic is unchanged. Verified: no PydanticDeprecatedSince20
warning fires for this validator anymore, and the existing test suite
still passes (5 passed).

Part of incremental migration for #256 - other validators/Field(example=)
usages in this file still need converting in follow-up PRs.

Signed-off-by: Brian Bergstrom <dulcetberg@gmail.com>

Author: dulcetberg

src/validation/models.py

@@ -24,7 +24,7 @@
 # Third party imports
 from geojson_pydantic import Feature, FeatureCollection, MultiPolygon, Polygon
 from pydantic import BaseModel as PydanticModel
-from pydantic import Field, validator
+from pydantic import Field, field_validator, validator
 
 # Reader imports
 from src.config import (
@@ -120,7 +120,8 @@ class Filters(BaseModel):
 
 
 class GeometryValidatorMixin:
-    @validator("geometry")
+    @field_validator("geometry")
+    @classmethod
     def validate_geometry(cls, value):
         """Validates geometry"""
         if value: