fix(rs): bump async-trait to 0.1.92, fix beta clippy double_must_use canary (LAB-2545)

[27Bslash6]

Summary

• The beta clippy canary has been red on every branch since 2026-08-30 with 9 double_must_use errors, all on the #[async_trait]-annotated Backend / TtlInspectable / LockableBackend trait definitions in backend/mod.rs.
• Root cause: async-trait <=0.1.91 unconditionally injects #[must_use] onto every generated trait method; clippy's beta double_must_use lint flags this as redundant since the method already returns a must_use Result. Upstream fixed this exact issue in 0.1.92 ("Resolve double_must_use clippy lint in generated code #303").
• No source changes needed: Cargo.toml already permits 0.1.92 under its "0.1" constraint. This PR just repins Cargo.lock (cargo update -p async-trait --precise 0.1.92).

Note: the ticket's original triage (grooming addendum) pointed at 13 explicit #[must_use] attributes on builder structs/accessors elsewhere in src/ as the suspected sites. Re-running cargo +beta clippy locally against the exact CI feature set showed none of those trigger the lint — the actual 9 errors all trace to the three #[async_trait] macro invocations in backend/mod.rs, confirmed byte-for-byte against the linked CI job log. Left those 13 sites untouched since they aren't the cause and aren't broken.

Test plan

• cargo +beta clippy --all-targets --features "cachekitio,redis,encryption,l1,macros,memcached,file" -- -D warnings — clean
• cargo +stable clippy (same flags) — clean
• cargo +1.85 clippy (same flags) — clean
• cargo +stable test --features "cachekitio,redis,encryption,l1,macros,memcached,file" — all pass (incl. doctests)
• cargo +stable fmt --all -- --check — clean
• cargo deny check — advisories/bans/licenses/sources all ok

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