test: cover protocolConstants service env parsing and cache

[Anichris-koded]

Closes #761

Summary

Adds tests/lib/backend/protocolConstants.service.test.ts with 35 unit tests targeting the service directly (not through the route).

What's tested

• Defaults – all fields (protocolVersion, network, fees, penalty tiers, commitment limits, cachedAt) when no env vars are set
• Env overrides – every env var the service reads, including float parsing, ACTIVE_CONTRACT_VERSION fallback precedence, and COMMITLABS_PENALTY_TIERS_JSON (with auto-generated description)
• Invalid env fallbacks – non-numeric values fall back to defaults; malformed/non-array/missing-field JSON tiers throw descriptive errors
• Caching – same object reference on repeated calls; env changes after first call are ignored; invalidateProtocolConstantsCache() forces re-read and new cachedAt

Coverage

100% statements, branches, functions, and lines on src/lib/backend/services/protocolConstants.ts (requirement ≥ 95%).

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[1nonlypiece]

good, covering the protocolConstants env parsing closes a real gap. merged.

[1nonlypiece]

good, covering the protocolConstants env parsing closes a real gap — landed locally since it touches a workflow file. merged.