[C9] `tests/` is 223 flat files with a single fixture-less conftest

[DsThakurRawat]

Labels: testing, tech-debt, priority:low
File: tests/ (223 flat test_*.py, subdirs only integration/ (2) + fixtures/cloud_burst/), tests/conftest.py (26 lines, only sets env, zero fixtures)

Description. No shared fixtures; common setup (temp bundles, fake ONNX, CLI runners, mock receipts) is re-implemented ad hoc across 223 files; flat layout makes targeting subsets hard.

Tasks.

• Add a fixture library to conftest.py (CLI runner, tmp bundle, receipt loader, fake-model factories).
• Group into tests/unit/, tests/integration/, tests/receipt/ with markers.

Dedup. #57/#58 add specific test cases; this is structural. Distinct.