LAB-411: fix lock acquire wire contract — snake_case, not camelCase

[27Bslash6]

Closes LAB-411.

Problem

CachekitIO::acquire_lock could never succeed against a spec-compliant CacheKit SaaS server. Both LockAcquireRequest and LockAcquireResponse were annotated #[serde(rename_all = "camelCase")], but the wire contract (protocol/spec/saas-api.md Lock Endpoints) is snake_case:

• Request — rs sent {"timeoutMs": N}; the server reads body.timeout_ms → undefined → NaN lock expiry.
• Response — the server returns {"lock_id": "<uuid>"} (acquired) or {"lock_id": null} (contested, protocol#22), always HTTP 200. serde expected lockId, and deny_unknown_fields turned the mismatch into a hard parse error — so every acquire failed, acquired or contested alike.

cachekit-py and cachekit-ts both handle snake_case correctly; rs was the lone outlier. No existing test exercised the acquire JSON round-trip, which is how it slipped through.

Fix

• Drop rename_all = "camelCase" from both structs — the Rust field names (timeout_ms, lock_id) are the wire names, so no rename attribute is needed at all.
• Keep deny_unknown_fields on the response (correct once the field name matches).
• Pin the contract with round-trip tests: acquired → Some(uuid), contested → None, and the literal serialized request body {"timeout_ms":5000}. These fail loudly if a rename ever reappears.

Verification

• New tests fail on the old code with exactly the diagnosed errors (unknown field 'lock_id', expected 'lockId'; body {"timeoutMs":5000}), pass after the fix.
• Full cargo test green, cargo clippy --all-targets clean, cargo fmt --check clean.

Found during LAB-240 SDK verification (contested-lock spec amendment, cachekit-io/protocol#22). Related: cachekit-ts#70.

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📥 Commits

Reviewing files that changed from the base of the PR and between e86172b and 5b368df.

📒 Files selected for processing (1)

• crates/cachekit/src/backend/cachekitio_lock.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch lab-411-lock-wire-snake-case

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