[blob-store·tests] pytest coverage for the publish logic (#2)

[chondl]

Track 2 · companion tests for #2 (bucket-first-serving). Base: bucket-first-serving; merge after #2. Test infrastructure only (backend/tests/, pytest.ini, requirements-dev.txt) — no production code changes. Optional.

Note: this branch was cut from an earlier commit of #2 and does not include #2's final commit (match-page-from-event-blob). It still merges cleanly onto #2 and only adds test files; rebasing onto #2's tip is a harmless cleanup.

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Follow-up to #2. Dependency-light pytest suite for the pure manifest/content-addressing module (src/google/publish.py): hash determinism, versioned/historical key formats, manifest round-trip, copy-on-write skip of unchanged blobs, carry-forward of unrendered manifest entries, and hist-epoch handling.

Kept separate from #2 because the project has no existing test infrastructure; this branch adds backend/tests/, backend/pytest.ini, and backend/requirements-dev.txt only.

cd backend
pip install -r requirements-dev.txt
python -m pytest tests/ -q   # 9 passed

[chondl]

Superseded by #19 — reopened as #19 [09] on cph-bucket-first-serving-tests.