Use job dependency flow for CI jobs

The Ruby SDK cleaned up their CI flow by using job dependency flows:

@neel.shah said:

> btw probably interesting for everyone, @peter made the ruby CI really clean by creating a job dependency flow
> it looks like this
>
> [https://github.com/getsentry/sentry-ruby/actions/runs/15043844757](https://github.com/getsentry/sentry-ruby/actions/runs/15043844757)
> the main file is this
> [https://github.com/getsentry/sentry-ruby/blob/master/.github/workflows/tests.yml](https://github.com/getsentry/sentry-ruby/blob/master/.github/workflows/tests.yml)
> that invokes each specific test matrix
> [https://github.com/getsentry/sentry-ruby/blob/master/.github/workflows/sentry_ruby_test.yml](https://github.com/getsentry/sentry-ruby/blob/master/.github/workflows/sentry_ruby_test.yml)

We could implement something similar for the Python SDK to better group tests. We already have some grouping, but this grouping basically puts all tests for different categories in different jobs. Instead, we could have a single job for tests, with all the current groups as sub-jobs of that main test job

Start by comparing the linked sentry-ruby .github/workflows/tests.yml and sentry_ruby_test.yml files, then inspect the Python SDK's existing CI grouping. Done means the current test categories run as sub-jobs under one parent test job, with the existing test coverage preserved.

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