systemtests: add per-test max_time override for precice-config

[AdityaGupta716]

Adds per-test max_time override in tests.yaml so individual tests can cap preCICE simulation time without manually editing precice-config.xml.
What changed:

Systemtest: max_time is a proper dataclass field (not set externally after construction)
__apply_precice_max_time_override() applies the override after copying the tutorial, before running — works for both run() and run_for_reference_results() so reference generation is consistent with test runs
Handles multiple tags with a warning instead of silently ignoring them
TestSuite parses max_time from tests.yaml and passes it through everywhere including generate_reference_results.py

Usage:
yamlopenfoam_adapter_pr:
tutorials:
- path: perpendicular-flap
case_combination:
- fluid-openfoam
- solid-calculix
reference_result: ./perpendicular-flap/reference-results/...
max_time: 0.01

Closes #402

[AdityaGupta716]

@MakisH Plz review

[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