[chore] Address TODO to migrate to VCRpy and remove bespoke RequestMocker code in Google GenAI SDK instrumentation

[michaelsafyan]

Description

Revises the tests to split up into two different categories:

1. A majority of tests that fully mock out the GenAI SDK (and do not need any request-level mocking).
2. An end-to-end ("e2e") test that uses VCRpy to test with a non-mocked GenAI SDK to sanity-check the system.

This addresses prior pull request feedback expressing concern regarding bespoke request mocking and deviation from typical pattern elsewhere of using the VCRpy component. This also addresses the original underlying concerns regarding velocity; since most tests do not use VCRpy and directly mock the underlying GenAI SDK, most feature changes can be tested with much simpler mocks of the GenAI SDK outputs.

Type of change

• Code cleanup

How Has This Been Tested?

• Recorded with:

• tox -e py312-test-instrumentation-google-genai-oldest --vcr-record=all

• Ran the tests with:

• tox -e py312-test-instrumentation-google-genai-oldest

Does This PR Require a Core Repo Change?

• No