fix: suppress trivial_numeric_casts in derived Encode/Decode

[Tyagiquamar]

Summary

• Derived Encode/Decode for enums emitted (index) as usize / as u8 even when the index was already that type, so crates with #![deny(trivial_numeric_casts)] failed to compile a two-variant enum.
• Allow trivial_numeric_casts on the generated index checks and match arms, matching the existing clippy::unnecessary_cast allows.
• tests/clippy.rs now denys that lint and includes a unit enum matching the report.

Test plan

• cargo +stable test --locked --test clippy
• cargo +stable clippy --locked --test clippy -- -Dwarnings
• CI fmt / clippy / tests jobs

Fixes #740

[Tyagiquamar]

@bkchr when you have a chance, could you take a look at this? It addresses #740 by allowing trivial_numeric_casts on generated enum index checks in derive(Encode, Decode), with tests ensuring crates with #![deny(trivial_numeric_casts)] compile cleanly. Happy to adjust anything needed.