test: import qiskit eagerly in conftest to fix test_simulation isolation failures

[ddri]

Summary

Fixes the qiskit half of #47: tests/test_simulation.py fails 2 tests when the file is run in isolation (e.g. pytest tests/test_simulation.py), while passing in full-suite runs. (The full-suite segfault part of #47 was a separate native-reload bug, already fixed by #46.)

Root cause

TestSimulationExecutor runs execute() inside patch.dict("sys.modules", {"qristal": ..., "qristal.core": ...}). Inside that block, circuit.to_openqasm() triggers qiskit's first import (from qiskit import qasm2, qasm3). qiskit loads its standard-gate library lazily, so a batch of qiskit.* submodules get added to sys.modules during the block — and patch.dict deletes them on exit because they weren't in the entry snapshot. qiskit is left half-initialized, so the next gate-class construction fails:

TypeError: invalid input: Instruction(name='rcccx_dg', num_qubits=4, num_clbits=0, params=[])

In full-suite runs an earlier file imports qiskit cleanly first, so the in-block import is a no-op and the failure is masked — which is why it only shows up in isolation.

Fix

Import qiskit once at collection time in tests/conftest.py, before any test can enter a patched sys.modules context. Guarded with try/except ImportError since qiskit is an optional dependency.

Verification

• pytest tests/test_simulation.py (isolation): 2 failed → 39 passed
• pytest tests/ (full suite): 414 passed, 16 skipped, exit 0 (unchanged; no regression)

Closes #47.

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Note

Low Risk
Test-only collection-time import with optional-dependency guard; no production or security impact.

Overview
Fixes isolated failures in tests/test_simulation.py (issue #47) by eagerly importing qiskit and qasm2/qasm3 at pytest collection time in tests/conftest.py.

When SimulationExecutor tests run inside patch.dict("sys.modules", ...), a first-time lazy qiskit import can add submodules that patch.dict removes on exit, leaving qiskit half-initialized and causing gate/Instruction errors on later use. Full-suite runs often mask this because another test imports qiskit first.

The new imports are wrapped in try/except ImportError so optional qiskit absence does not break collection.

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