FlightData: add mass storage reboot action - #3756

Automated review note — AI-generated (Claude), validated against the live diff. Please sanity-check before acting.
Full report: https://uav.tridgell.net/DevCallReviews/2026_08_30/devcall_pr_reviews.html#prMissionPlanner-3756

Reviewed at head cfc80d28c2. APPROVE — no blockers. What was checked: the new handler sits inside the standard "Are you sure" confirmation gate and follows the neighbouring disable/doCommand/re-enable/return pattern exactly; param1=5 matches the mass-storage action from the companion mavlink change implemented by ardupilot#34087 (linked in the body), and older firmware rejects it safely; the mid-enum insertion is safe in-tree (the action combo is built from Enum.GetNames, all dispatch compares by name, nothing persists numeric values). An independent Codex validation and a cold second review both ran.

One thing worth a line in the commit message: the TryGetValue refactor fixes a real bug on master — CustomActions is a Dictionary<string, Action<string>>, so master's CustomActions[CMB_action.Text] throws KeyNotFoundException on every built-in action click (only the global WinForms error-report handler catches it).

Non-blocking notes:

1. FlightData.actions is public, so inserting mid-enum shifts later constants for any precompiled external plugin that baked them in. No in-tree numeric consumer exists, but appending the member at the end removes the edge for free (FlightData.cs:190).
2. doCommand() special-cases PREFLIGHT_REBOOT_SHUTDOWN as fire-and-forget (never reads the ACK), so a rejection — old firmware, armed — is silent. Consistent with the existing Preflight_Reboot_Shutdown semantics; a "may be unsupported on this firmware" hint in the confirmation text would help while ardupilot#34087 is unreleased.