Fix flaky telephony pool tests

[mitchellhenke]

🛠 Summary of changes

I think #7314 introduced some mutation of the constant that can affect other tests and may not always clean it up which can lead to failed specs elsewhere (example)

To reproduce the flaky test reliably, run rspec spec/lib/telephony/pinpoint/sms_sender_spec.rb spec/lib/telephony/telephony_spec.rb --seed 8116, which should result in:

Failures:

  1) Telephony.phone_info with pinpoint adapter uses the pinpoint adapter
     Failure/Error:
       response = client.phone_number_validate(
         number_validate_request: { phone_number: phone_number },
       )

     NoMethodError:
       undefined method `phone_number_validate' for #<Pinpoint::MockClient:0x00007f94fb854e30 @config=#<struct Telephony::PinpointSmsConfiguration application_id="fake-pinpoint-application-id-sms", shortcode="123456", country_code_longcode_pool={"PR"=>["+19393334444"]}, region="fake-pinpoint-region-sms", access_key_id="fake-pnpoint-access-key-id-sms", secret_access_key="fake-pinpoint-secret-access-key-sms", credential_role_arn=nil, credential_role_session_name=nil, credential_external_id=nil>>

This PR tries to ensure that all of the client pool mocking is reset in this spec file.

[zachmargolis]

LGTM, good catch, thanks!