fix: migrate GeometryValidatorMixin to Pydantic V2 field_validator

[dulcetberg]

Summary

First incremental step on #256 — migrates GeometryValidatorMixin's geometry validator from the deprecated Pydantic V1 @validator API to V2's @field_validator, adding the explicit @classmethod that V2 requires (V1 handled this implicitly).

Validator logic itself is unchanged — only the decorator mechanics.

This repo currently resolves Pydantic 2.12.3, and the old @validator syntax still works but raises PydanticDeprecatedSince20 warnings, with removal planned for Pydantic V3. This PR is scoped to just this one validator, per the plan discussed in the issue — 8 more @validator usages remain in src/validation/models.py for follow-up PRs, along with a related Field(..., example=...) → Field(..., json_schema_extra={...}) deprecation pattern noticed while testing.

Test plan

• Verified no PydanticDeprecatedSince20 warning fires for this validator after the change (previously did)
• pytest tests/test_app.py — 5 passed, 0 failed (remaining warnings are all from the other, not-yet-migrated validators/Fields in this file, unrelated to this change)
• Confirmed the field_validator + classmethod pattern works correctly when defined on a mixin combined via multiple inheritance (RawDataCurrentParamsBase(BaseModel, GeometryValidatorMixin)), matching how this class is actually used

Closes part of #256 (first of several incremental PRs).

[spwoodcock]

Thanks for starting this =)

On a basic search I see another usage of @validator: https://github.com/search?q=repo%3Ahotosm%2Fraw-data-api+%40validator&type=code

Pydantic v2 also deprecates usage of class Config in favour of a new syntax: https://github.com/search?q=repo%3Ahotosm%2Fraw-data-api+class+Config&type=code

There may be other things too

[dulcetberg]

I already scoped the remaining validators (lines 207, 236, 333, 372, 431, 452, 558, 674) and the 4 class Config blocks that need converting to model_config. Planning to keep working through them as small incremental PRs, same as this one.

[spwoodcock]

See hotosm/drone-tm#872 (comment)