Fix Artifice.deactivate to properly restore Gem::Net::HTTP

The hardcoded Gem::Net::HTTP in Artifice.deactivate, is actually
the replaced Artifice::Net::HTTP. This doesn't restore the original
Gem::Net::HTTP.

Restore the saved original Gem::Net::HTTP in Artifice.deactivate

This issue caused the tests sometimes failed in
spec/bundler/fetcher/gem_remote_fetcher_local_ssl_server_spec.rb
using Artifice::Net::HTTP unintentionally. Because localhost is not included in
spec/support/artifice/helpers/endpoint.rb - permitted_hosts caused HTTP status 403
(response.body: "Host not permitted"). But Gem::Net::HTTP should be used in the
tests instead of Artifice::Net::HTTP. Possibly this issue happens when
Artifice.deactivate is called in other tests such as spec/commands/ssl_spec.rb.
That's why this issue sometimes happened rather than always.

```
  1) Bundler::Fetcher local SSL server #connection PQC connects
     Failure/Error: expect(response.code).to eq("200")

       expected: #<Encoding:UTF-8> "200"
            got: #<Encoding:ASCII-8BIT> "403"

       (compared using ==)
     # ./spec/bundler/fetcher/gem_remote_fetcher_local_ssl_server_spec.rb:49:in 'block (4 levels) in <top (required)>'
     # ./spec/spec_helper.rb:164:in 'block (4 levels) in <top (required)>'
     # ./spec/spec_helper.rb:164:in 'block (3 levels) in <top (required)>'
     # ./spec/support/helpers.rb:414:in 'block in Spec::Helpers#with_gem_path_as'
     # ./spec/support/helpers.rb:428:in 'Spec::Helpers#without_env_side_effects'
     # ./spec/support/helpers.rb:409:in 'Spec::Helpers#with_gem_path_as'
     # ./spec/spec_helper.rb:163:in 'block (2 levels) in <top (required)>'
     # ./lib/rubygems.rb:306:in 'Kernel#load'
     # ./lib/rubygems.rb:306:in 'Gem.activate_and_load_bin_path'
```

spec/commands/ssl_spec.rb
```
...
  after(:each) do
    ...
    Artifice.deactivate
    ...
  end
...
```

Co-Authored-By: Claude <noreply@anthropic.com>

Author: junaruga

spec/support/artifice/helpers/artifice.rb

@@ -12,13 +12,15 @@ module Artifice
   def self.activate_with(endpoint)
     require_relative "rack_request"
 
+    @original_net_http = ::Gem::Net::HTTP
     Net::HTTP.endpoint = endpoint
     replace_net_http(Artifice::Net::HTTP)
   end
 
   # Deactivate the Artifice replacement.
   def self.deactivate
-    replace_net_http(::Gem::Net::HTTP)
+    replace_net_http(@original_net_http) if @original_net_http
+    @original_net_http = nil
   end
 
   def self.replace_net_http(value)