change linear solver updater to 25.9.1

[awccoppFC]

Note

Medium Risk
Adjusts version-migration behavior for SimulationParams by introducing a new milestone; incorrect migration could break loading/validation of older JSONs that omit the linear solver discriminator.

Overview
Adds a new 25.9.1 migration step that populates missing type_name: "LinearSolver" under models[*].navier_stokes_solver.linear_solver, and registers it in VERSION_MILESTONES so the main updater() can move 25.9.0 -> 25.9.1.

Updates _to_25_9_0 to stop after removing the deprecated meshing key and migrating wall-function booleans, and adds unit tests covering the new updater function, end-to-end update path, and no-models edge cases.

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