systemtests: fix Python bindings version + robustness (fixes #584)

[PranjalManhgaye]

Description

Problem
Tutorial run.sh scripts create their own .venv and install from requirements.txt (e.g. pyprecice~=3.0), silently overwriting the specific PYTHON_BINDINGS_REF version installed by the Docker image. This means the version argument passed to system tests was completely ignored.

Root cause
The Docker image installs the correct version into /home/precice/venv but never permanently activates it. At container runtime the venv isn't on PATH, so the run script sees no active venv and creates its own. PRECICE_TUTORIALS_NO_VENV (added in #680) was never set in the component templates, so the escape hatch was never triggered.

Fix

• Add ENV VIRTUAL_ENV and ENV PATH after venv creation in python_bindings, fenics_adapter, and nutils_adapter Dockerfile stages — permanently activating the correct venv at container runtime
• Set PRECICE_TUTORIALS_NO_VENV=1 in python-bindings, fenics-adapter, and nutils-adapter component templates — so run scripts skip creating their own venv and use the Docker image's version
• Add sanity check: before running, verify precice is importable when PRECICE_TUTORIALS_NO_VENV is set; fail fast with a clear error instead of a confusing failure later
• Changelog: note that ubuntu_2204 uses pip install --user (no venv) and is fixed by the template change alone

Fixes #584