upgrade mqtt5 to 0.39.2 from crates.io and pin rust toolchain to 1.98.0

[fabracht]

Summary

• Upgrades mqtt5 from a git dependency (0.35.1) to the published crate 0.39.2 (pulling mqtt5-protocol 0.15.0), and removes the [patch.crates-io] git override so every build resolves mqtt5 from crates.io.
• The upgrade is API-compatible for everything MQDB uses (client, broker config/auth/events/bridge/acl, message types) — no source changes were needed.
• mqtt5 0.39 adds user_id to the broker's client connect/disconnect events, which the upcoming presence feed depends on.
• Pins the Rust toolchain to 1.98.0 via rust-toolchain.toml (matching the mqtt5 library) and changes CI from dtolnay/rust-toolchain@stable to @1.98.0, so local, CI, and the upstream library build on the same compiler and the local-vs-CI clippy skew is gone.
• Bumps the four crates that link mqtt5: agent 0.8.27, cluster 0.4.13, vault 0.1.5, cli 0.8.36.

Test plan

• cargo make clippy — zero warnings across the workspace and the wasm target
• cargo test — all suites pass against mqtt5 0.39.2
• cargo check --workspace --all-features --all-targets compiles
• cargo make format-check clean