control: add guard rails in flight control paths

[vlordier]

This draft isolates a set of control-path robustness changes from the larger hardening branch.

Summary:

• add arithmetic / finite-value guards in Copter, L1, AP_Math, and AP_Motors control paths
• add focused AP_Math regression coverage for invalid accel-limit inputs and extreme gain / denominator edge cases

Why:

• these changes were originally buried inside a much larger branch
• this draft allows review of the control-path behavior changes on their own

Validation:

• branch was split cleanly from upstream/master
• the branch includes focused AP_Math regression test updates for the edge cases covered by this slice
• not fully validated locally as a standalone branch beyond the split and commit flow
• draft only: this slice includes behavior-affecting control changes and needs careful review

[IamPete1]

If you have evidence that this can actually happen we should fix that not try and deal with it here.