Fix MCQ qualifier constraint parsing

[cbizon]

Summary

• Parse TRAPI constraints.qualifiers in addition to qualifier_constraints.
• Reuse one QEdge qualifier parser for MCQ and single-curie query paths so qualifier filters are constructed consistently.
• Add regression coverage for issue QEdge qualifier constraints are not applied as a strict filter for set-input affects queries #141 qualified biolink:affects MCQ queries.
• Make pytest import this repo's local src package instead of any globally injected src package.
• Split test-only dependencies into requirements-test.txt and document conda/venv-based installs.

Why

• Qualified set-input queries were preserving qualifier constraints in the echoed query graph but passing only the bare predicate into graph coalescence, so missing, broader, or opposite qualifier rows could pass through.
• This repo uses src as its top-level import package, but src lacked an init.py and pytest had no local pythonpath config. On a machine with another installed src package, normal pytest could import the wrong repo.
• Deployment installs requirements.txt, so pytest dependencies should not be runtime requirements. Local installs and CI now use an explicit conda or virtual environment rather than system Python.

Test

• python -m venv /private/tmp/ac-test-env-answercoalesce-141
• /private/tmp/ac-test-env-answercoalesce-141/bin/python -m pip install -r requirements-test.txt
• /private/tmp/ac-test-env-answercoalesce-141/bin/python -m pytest tests/test_trapi_generation.py

Fixes #141

[wumirose]

Looks good to me