systemtests: fix Python bindings version ignored due to tutorial venv (fixes #584)

[AdityaGupta716]

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, nutils-adapter, and su2-adapter component templates
Add a sanity check that fails fast with a clear error if precice is not importable when the escape hatch is set
Fix --system-site-packages typo in flow-over-heated-plate/fluid-su2/run.sh

Fixes #584

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/gsoc-2026-aditya-gupta/2773/4

[AdityaGupta716]

@MakisH the component templates were never setting PRECICE_TUTORIALS_NO_VENV=1 (the escape hatch added in #680), and the Docker stages weren't permanently activating /home/precice/venv at runtime. This PR closes both gaps for python_bindings, fenics_adapter, nutils_adapter, and su2_adapter also adds a fast-fail sanity check if precice isn't importable when the escape hatch is set, and fixes a system-site-packages typo in flow-over-heated-plate/fluid-su2/run.sh. Please review!