[FXC-4069] Allow AutomatedFarfield with CustomVolume

[alexxu-flex]

Enables using AutomatedFarfield with CustomVolumes.
The user must define enclosed_surfaces, a list of user-defined patches (not RotationVolumes, etc.) that defines the exterior farfield volume along with the AutomatedFarfield.farfield. Translation turns this into a "farfield" zone, which is then handled on the mesher side.
Validation is updated to enforce enclosed_surfaces iff CustomVolumes. Updated unit tests check this and block "farfield" name collision.

Coming (soon?) in a separate PR: expose _FarfieldBase.zone to the user (requiring enclosed_surfaces with CustomVolumes), and relaxing the restriction on needing to specify a UserDefinedFarfield zone if CustomVolumes are specified.

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Note

Medium Risk
Touches meshing parameter validation and JSON translation, which can change accepted configs and the solver/mesher contract; risk is mitigated by added unit tests covering key positive/negative cases.

Overview
Enables using AutomatedFarfield together with CustomVolume zones by introducing AutomatedFarfield.enclosed_surfaces and translating it into an explicit generated "farfield" zone.

Adds cross-field validation to enforce enclosed_surfaces iff CustomVolumes are present, and prevents name collisions by reserving CustomVolume name "farfield" (and disallowing enclosed_surfaces containing a surface named "farfield"). Also loosens the beta-mesher farfield compatibility check to allow automated ("auto") farfield for CustomVolume.

Updates/extends unit tests for both validation and translator output to cover the new workflow and error cases.

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