Fix nightly ICE: prevent tokio rt-multi-thread from being activated via --features srv

[Copilot]

cargo test --features srv fails on nightly rustc (1.99.0-nightly, 2026-07-24) with an internal compiler error in rustc_codegen_ssa when compiling tokio-1.53.1 with the rt-multi-thread feature.

Root cause

rt-multi-thread is pulled in transitively:

whois-rust (--features srv)
  → hickory-client 0.24.4 (default features)
    → hickory-proto 0.24.4 (feature: tokio-runtime)
      → tokio/rt-multi-thread

hickory-client and hickory-proto both use default-features = true on their tokio dependency, and hickory-proto's tokio-runtime feature explicitly enables tokio/rt-multi-thread.

Changes

• Set default-features = false on the hickory-client dependency in Cargo.toml to prevent hickory-proto's tokio-runtime feature from activating tokio/rt-multi-thread unnecessarily when only the synchronous SRV DNS lookup path is used.