Require outer script reference in Mock._init to remove class name dependency

[ChrisCrossCrash]

Summary

Mock.install() and Mock.uninstall() previously referenced the C3Http identifier directly. This breaks when class_name C3Http is commented out to avoid global scope pollution — a common pattern for addon authors who want to prevent the class from appearing in every project that includes the addon.

Mock now accepts the outer GDScript reference in _init and stores it, so install/uninstall use _outer._impl instead of C3Http._impl. With this change, commenting out class_name C3Http in the addon file requires no further changes to the file.

Breaking change: Mock.new() → Mock.new(C3Http). All internal call sites are updated.

Usage

When class_name C3Http is active (the default):

mock = C3Http.Mock.new(C3Http)
mock.install()

When class_name C3Http is commented out, load the script into a local constant and pass it through:

const C3Http := preload("res://addons/c3-http-request/c3_http_request/c3_http_request.gd")

mock = C3Http.Mock.new(C3Http)
mock.install()

install() and uninstall() remain argument-free — the reference is captured once at construction.

Test plan

• All 286 GUT tests pass (godot --headless --path . -s addons/gut/gut_cmdln.gd -gdir=res://tests -ginclude_subdirs -gexit)
• Manually verify that commenting out class_name C3Http and using the preload pattern produces no GDScript errors