feat(WIP): allow pytest deps to come from target resolve

[lecardozo]

Context

Currently, the pytest runner creates an environment composed by a mix of the python_test dependencies and the pytest dependencies that are needed for the test to properly run. In cases which a third-party dependency is contained both the python_test dependency as well as pytest with potentially different versions (since they come from independent resolves), this can lead to inconsistencies between production and testing environment, which is undesirable.

Related issues and threads

• Allow Python tools to install from a different resolve/lockfile per-process, instead of a global static resolve #18532 (comment)
• https://pantsbuild.slack.com/archives/C046T6T9U/p1761670638052709

Potential solution (work in progress)

One way to avoid that is to make sure pytest dependencies come from the same resolve as the one used by the python_test target. In cases where the monorepo has a single resolve, this can already be solved with the pytest.install_from_resolve setting. However, in cases where the monorepo contains more than one resolve, there's currently no way to solve that.

This PR introduces pytest.install_from_target_resolve: BoolOption setting that, when set true, ignores the pytest resolve configuration and expects test dependencies to come from the target resolve. This ensures all dependencies are aligned across different execution environments.

Potential problems

• Since we are ignoring pytest dependencies, we need to make sure they are included on the list of dependencies before building the venv, otherwise this won't work. this should be addressed on 903daaa

[lecardozo]

author's note: maybe this logic would be better close to PythonToolBase.pex_requirements

pants/src/python/pants/backend/python/subsystems/python_tool_base.py

Line 254 in 1a7da5c

def pex_requirements(

Kept it here initially because it feels like this behavior is only desirable for pytest specifically not all python tools due to it's nature as both a library and a tool

[benjyw]

We can always move it later, and for now this keeps the blast radius more contained.

[benjyw]

This looks good, thanks for the contribution!

How have you tested it? It would be good to add some testing in pytest_runner_test.py.