bug(FXC-5154): add function to compute min radius

[marc-flex]

Note

Low Risk
Localized refactor affecting only pre-simulation geometry validation and warning messages; no solver/runtime behavior changes beyond when warnings are emitted.

Overview
Refactors the small-cylinder-radius validation in HeatChargeSimulation by introducing _get_cylinder_radii_with_meshing_tol() to compute/clamp cylinder radii to a minimum meshing-safe value.

Replaces the previous inline min_radius computation (formerly tied to OPENCASCADE_* naming) with CYLINDER_RADIUS_TOL and updated warnings that trigger when radii are clamped (including handling of negative/too-small radii).

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[Copilot]

Pull request overview

This PR refactors cylinder radius validation logic in the TCAD heat simulation module by extracting duplicate code into a reusable helper function. The changes improve code maintainability while preserving existing functionality for handling cylinders with radii that are too small for meshing.

Changes:

• Renamed constant OPENCASCADE_CYLINDER_RADIUS_TOL to CYLINDER_RADIUS_TOL for better generality
• Extracted minimum radius computation and clamping logic into new helper function _get_cylinder_radii_with_meshing_tol
• Refactored _warn_small_cylinder_radius validator to use the new helper function instead of duplicating logic

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[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/tcad/simulation/heat_charge.py (100%)

Summary

• Total: 12 lines
• Missing: 0 lines
• Coverage: 100%