[Hotfix Main]: feat(BETDisk): add collective_pitch field [SCFD-7370]

[github-actions]

Hotfix of PR #1938 (#1938) to the main branch.
Hey @benflexcompute, please review this hotfix PR created from your original PR.

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Note

Medium Risk
Medium risk because it changes BETDisk-to-solver translation by modifying all translated twist angles, which can affect aerodynamic results; behavior is backward-compatible when collective_pitch is unset (treated as 0).

Overview
Adds an optional collective_pitch angle to BETDisk (and its input cache) and wires it through all from_* constructors.

Updates the solver translator so collective_pitch is applied as a uniform degree offset to every translated twists[*].twist value (defaults to 0 when unset), and extends reference JSON fixtures plus new tests to verify input-cache persistence and translation behavior/serialization when collective_pitch is None.

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