Improve test suite runtime

[Copilot]

The test suite had redundant per-test overhead from repeated force field parsing, unnecessary use of heavyweight code paths, and suboptimal CI configuration.

Changes

Fixture consolidation

• Replaced xml_ff_bo_bonds and xml_ff_bo (raw XML strings requiring ForceField(sage, xml_str) reconstruction in every test) with sage_bo_bonds and sage_bo_both fixtures that return fully-combined ForceField objects once

Avoid Interchange where it's not under test

• test_fractional_bondorder_from_molecule now calls forcefield.create_openmm_system() directly instead of converting through Interchange.from_smirnoff() → to_openmm(), removing a heavyweight round-trip that wasn't exercising anything under test

Reduce charge assignment cost

• Switched test_virtual_site_type_stored_in_potential_key from sage (triggers NAGL charge assignment) to sage_no_nagl (falls back to cheaper formal charge method) since the test is about virtual site key types, not charge values

CI / runner configuration

• Decoupled postinstall (pip install -e .) into a dedicated CI step rather than a depends-on on every pixi task, avoiding redundant re-installs
• Changed -n logical → -n auto to use all available hardware threads
• Set fail-fast: false so matrix jobs run to completion and provide full signal on failures

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