fix(kura): return JSON for cache miss responses

Summary

• Return JSON 404 error bodies for Kura key-value cache misses instead of empty responses.
• Return JSON 404 error bodies from the shared artifact GET path, covering CAS, module cache, and Gradle cache misses.
• Add HTTP route tests that assert the miss responses include application/json bodies so generated clients do not fail with missing response-body errors.

Testing

• mise exec -- cargo fmt
• mise exec -- cargo test misses_return_json
• mise exec -- cargo test clean_namespace_removes_existing
• mise exec -- cargo clippy --all-targets -- -D warnings

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