Independent genai versioned packages are tested with monorepo dependencies

We have independent releases for some genai packages, but the tox setup in this repo still installs monorepo deps for testing. For example:

• opentelemetry-instrumentation-google-genai depends on
  • opentelemetry-instrumentation
  • opentelemetry-util-genai

but the requirements files that tox uses installs those from the monorepo (for both oldest and latest) https://github.com/open-telemetry/opentelemetry-python-contrib/blob/a025a206da0278c39cd3a7c5014a48e129bd6827/instrumentation-genai/opentelemetry-instrumentation-google-genai/tests/requirements.oldest.txt#L33-L37

This is problematic because those packages are not released at the same time, so the monorepo version is not available after publishing. The tests don't reflect the reality of what users see after releasing. However it's convenient for development, for example making an update to opentelemetry-util-genai and opentelemetry-instrumentation-google-genai at the same time. It also makes the release painful (see https://github.com/open-telemetry/opentelemetry-python-contrib/pull/4230). Some possible options to fix it

1. Strictly only test against published versions (for both oldest and latest). Never use any monorepo deps for independent versioned packages. The downside is that it's hard to develop new features in opentelemetry-util-genai without releasing it after every change.
2. Do away with independent versioned pacakges. Go back to strict lockstep releases of contrib.
3. Live with the testing not being accurate.
4. Test against monorepo deps on main, but release branches (created by package-prepare-release workflow) only test against published versions. Updating the package-prepare-release workflow to do this might be a little painful, but maybe solves all of the above?

Start by reading the linked requirements.oldest.txt entries and the .github/workflows/package-prepare-release.yml workflow, then review the release behavior described in pull request 4230. First clarify which testing policy is accepted; done means the chosen policy is reflected in tox and release-branch testing without relying on unavailable monorepo versions.