chore: override API mixins when needed (#21)

- [ ] Regenerate this pull request now.

PiperOrigin-RevId: 477248447

Source-Link: googleapis/googleapis@4689c73

Source-Link: googleapis/googleapis-gen@c405978
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiYzQwNTk3ODZhNWNkODA1YTAxNTFkOTViNDc3ZmJjNDg2YmNiY2VkYyJ9

test: use fully qualified request type name in tests
PiperOrigin-RevId: 475685359

Source-Link: googleapis/googleapis@7a12973

Source-Link: googleapis/googleapis-gen@370c729
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiMzcwYzcyOWUyYmEwNjJhMTY3NDQ5YzI3ODgyYmE1ZjM3OWM1YzM0ZCJ9

Author: gcf-owl-bot[bot]