fix: harden hooks, plugin lifecycle, and addon download failure handling

[M-archand]

Some hardening fixes for potential crashes, leaks, and retry loops.

Prevent unhandled exceptions (9373c06)

No try/catch anywhere despite doing unsafe pointer reads, Steam API calls, and per-client hook work. All 12 entry points now catch, log with context, and return a safe default.

Clean up plugin state on unload (3070cd1)

Unload() was empty and the ServiceProvider was never disposed. So leaked the Steam Callback<T> & the addon search paths added via AddSearchPath.

Check Steam error message lookup (85f3c1c)

SteamErrorMessage.Errors holds 133 entries and was indexed directly by the raw EResult, so any code Valve adds beyond that in the future will throw IndexOutOfRangeException inside the download failure handler.

Stop failed downloads from looping the map forever (29694f8)

OnAddonDownloaded cleared the addon from ImportantDownloads regardless of result, so a failure took the success path and called ReloadMap(), which raised OnStartupServer, re-ran RefreshAddons, re-queued the same addon, and failed again indefinitely. Failures are still retried on the next map load.