[daniella-techie]

Summary

Fixes how the Redis-backed idempotency response cache (src/common/middleware/idempotency.middleware.ts) handles corrupted entries.

Before this change, a corrupted entry was skipped but never removed. An entry that parsed as JSON but had the wrong shape (for example null, a missing body, or an invalid status) could also make the replay fail after headers had already been written.

Changes

• Catch JSON parse errors: parsing now happens in its own parseCachedEntry helper, and its result is checked before use: it must be an object with a body and an integer HTTP status from 100 to 599.
• Treat as a cache miss: an invalid entry returns false from replayCached, so the request goes through the normal lock-and-handle flow.
• Delete corrupted entries: the bad key is removed with DEL. If the delete itself fails, a warning is logged and the request still goes ahead. The handler's fresh 2xx response then replaces the entry.
• Tests: new cases cover invalid JSON, null, a missing body, a non-numeric status and an out-of-range status, plus a failing DEL.

Test plan

• npx jest src/common/middleware/idempotency.middleware.spec.ts: 10/10 passing

Closes #735
Closes #736
Closes #737
Closes #738

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