Harden Steam Input gamepad slot mapping

[Infiland]

Fixes #159.

Summary

• Reject invalid XInput slot values outside 0..3 in steam_input_get_controller_for_gamepad_index instead of forwarding them to Steam.
• Try Steam's reverse lookup in steam_input_get_gamepad_index_for_controller, then fall back to scanning slots 0..3 and comparing controller handles.
• Update the docs to state that invalid slots return 0 and unmapped handles return -1.

Validation

• git diff --check
• xcodebuild -list -project source/Steamworks_gml/extensions/steamworks/steamworks_macos/Steamworks.xcodeproj
• Checked Steamworks ISteamInput documentation for GetControllerForGamepadIndex and GetGamepadIndexForController behavior.

Notes

Native build verification is blocked in this checkout because source/Steamworks_sdk/steam_api.h is not present locally, and the normal macOS build also requires the project signing certificate.