test(cachekit): pin {"ttl": null} decoding of TtlResponse (LAB-3593)

[27Bslash6]

Summary

This PR adds a test to pin the JSON decoding behavior of TtlResponse when the server returns a ttl field with a null value (LAB-3593).

Changes

Adds a new unit test ttl_response_decodes_null_ttl_as_none in crates/cachekit/src/backend/cachekitio_ttl.rs that verifies three decoding scenarios:

• {"ttl": null} decodes successfully to None (representing a live key with no expiry), rather than producing a parse error.
• {"ttl": 42} decodes successfully to Some(42).
• {"ttl": null, "extra": 1} produces an error, confirming that unexpected/extra fields are rejected.

Purpose

The server responds with {"ttl": null} for live keys that have no expiry set. This test locks in the expected behavior that such a response is treated as a valid None value and not misinterpreted as a decoding failure, guarding against future regressions in TtlResponse deserialization.

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

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Walkthrough

The change adds unit coverage for TtlResponse deserialisation. The tests check null, numeric TTL values, and unknown response fields.

Changes

TTL response tests

Layer / File(s)	Summary
TTL deserialisation coverage crates/cachekit/src/backend/cachekitio_ttl.rs	Tests verify that null maps to None, numeric TTL values map to Some(value), and unknown fields are rejected.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to cec92

This PR adds focused TTL decoding coverage without changing production behavior, so it is mergeable after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies a regression test for TtlResponse TTL decoding and matches the pull request changes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit unit tests in branch lab-3593-ttl-null-test

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