fix: migrate 3 cross-field validators to Pydantic V2

[dulcetberg]

Summary

Converts the 3 remaining @validator usages in src/validation/models.py that read other fields via V1's values parameter (deliberately deferred out of #309 since they need the mode="before" + info.data treatment rather than a plain decorator swap):

• RawDataCurrentParams.check_bind_option (bind_zip)
• StatsRequestParams.set_geometry_or_iso3 (geometry)
• DynamicCategoriesModel.set_geometry_or_iso3 (geometry — this one also has extra HDX dataset/category cross-checks beyond the base geometry/iso3 logic)

@validator(field, allow_reuse=True) → @field_validator(field) + @classmethod; values.get(...) → info.data.get(...).

The always=True gotcha

The two set_geometry_or_iso3 validators used pre=True, always=True in V1, so they run even when geometry is omitted (this is how the "either geometry or iso3, not neither" check works). V2's field_validator has no always equivalent — mode="before" alone does not run when the field is omitted and its default is used. Verified this with an isolated test before relying on it.

The correct V2 replacement is validate_default=True on the field's own Field(...) declaration, added here to both geometry fields. Re-verified behaviorally across all 4 cases (geometry-only, iso3-only, both supplied, neither supplied) that this reproduces the original V1 semantics exactly.

Part of the pydantic v1→v2 migration tracked in #256. Continues the incremental approach from #307/#308/#309.

Test plan

• pytest tests/test_app.py passes (5 passed)
• No remaining deprecation warnings for these 3 validators
• Isolated behavioral test of StatsRequestParams: neither/both/geometry-only/iso3-only all raise/pass exactly as before
• Isolated behavioral test of RawDataCurrentParams.check_bind_option: disallowed output_type + bind_zip=False raises, allowed output_type + bind_zip=False passes, default bind_zip passes regardless of output_type
• Isolated unit test of DynamicCategoriesModel.set_geometry_or_iso3's extra HDX dataset-required-when-hdx_upload logic

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