Remove enclosed_entities from UDF

[alexxu-flex]

After #1860, some localTests broke because they needed enclosed_entities on dummy UserDefinedFarfields used with CustomVolumes.
This PR reverts #1889, which attempted to fix the above by allowing CVs to be used without UDF. We'll allow this later, but aren't confident we can support this yet (need: move domain_type from farfield to MeshingParams; check any places that may be broken by the lack of a farfield)

This PR also removes enclosed_entities from UDF, because it makes less sense to specify just the inner boundaries of the farfield when the outer one is not auto generated (unlike Auto or WT). To achieve the same functionality, currently, the user should create a CV with both the inner and outer boundaries of the farfield volume (mesher seems to silently lose the outer geometry otherwise?).

bonus barely-related fix: now that CustomZone can be used with WindTunnel, the WT zone is not necessarily at index 0

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Note

Medium Risk
Tightens meshing schema/validation around farfield/custom-volume setup and changes translation/validation behavior, which can break existing configs that relied on implicit farfield or UserDefinedFarfield.enclosed_entities. Scope is contained to meshing parameter models, translators, and related validators/tests.

Overview
UserDefinedFarfield no longer accepts enclosed_entities. Support for enclosed_entities is moved behind a new _FarfieldAllowingEnclosedEntities base used only by AutomatedFarfield and WindTunnelFarfield, and all validators/translators now only read/emit farfield patch data from those types.

Volume zone validation is tightened. MeshingParams.volume_zones now always requires an explicit farfield zone (no more CustomVolumes-only/implicit farfield), and farfield_method inference no longer treats CustomZones as implicitly user-defined in the non-modular workflow.

Validation/translation fixes and test updates. Wind-tunnel boundary collection no longer assumes the wind-tunnel zone is at index 0, farfield-enclosed-entity extraction skips UDF, and tests are updated/added to assert UDF rejects extra fields and that translations omit the synthetic farfield zone when no supported enclosed_entities are present.

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