CI: replace pixi with pip for test + code-quality jobs

[JanStreffing]

Problem

The Test Python Code and Code Quality Checks jobs both fail at the pixi install / pixi install -e dev step — i.e. environment resolution, before any project code runs. This is preexisting on master (unrelated to any code change).

Evidence it's pixi's resolver and not the project deps:

• The Build Documentation job, which installs the same package via plain pip install -e ., passes.
• The non-pixi checks (Documentation Check, Security Scanning, Trivy) pass.

Fix

Switch both jobs from setup-pixi to actions/setup-python + pip, mirroring the green docs job:

• test-python: install libeccodes-dev via apt (so the gribapi/eccodes Python bindings import — gribapi is the only conda-specific import, in lsm.py), then pip install the runtime deps actually imported by the tested modules. pyfesom2 is not imported at module top level by any of the tested modules, so the heavy git dependency is omitted. Then run the existing syntax + import checks unchanged.
• code-quality: pip install flake8 black isort and run them directly (these need no project deps).

Verification (local)

• All tracked Python compiles (py_compile) cleanly.
• The flake8 hard-fail gate --select=E9,F63,F7,F82 reports 0 over the tracked tree (flake8 7.3.0).
• ci.yml parses as valid YAML.

The black/isort steps remain non-blocking (|| echo ::warning::), exactly as before.

Note: this is orthogonal to #48 (the CO2/INIUA staging fix) — that PR's red checks are this same preexisting pixi breakage and can be merged independently once this lands and its branch is updated.