hardening: return Err instead of panic in ssz_decode_fixed_vec

[MegaRedHand]

Summary

• Both the trait default (SszDecode::ssz_decode_fixed_vec) and the derive-generated override called chunks_exact(0) when invoked on a variable-size type, causing an unconditional panic (division by zero in chunks_exact).
• This PR adds a new DecodeError::NotFixedSize variant and uses it to return Err instead of panicking in both code paths.
• All internal call sites already guard with is_fixed_size() before calling ssz_decode_fixed_vec, so this change is purely defensive. It protects against direct external misuse of the API.

Changes

File	Change
crates/ssz/src/error.rs	Add NotFixedSize variant to DecodeError
crates/ssz/src/decode.rs	Guard trait default with item_size == 0 check
crates/ssz-derive/src/lib.rs	Replace dead-code chunks_exact fallback with Err(NotFixedSize)

Test plan

• cargo build --workspace --all-targets --exclude spec-tests
• cargo clippy --workspace --all-targets --exclude spec-tests -- -D warnings
• cargo test --workspace --exclude spec-tests
• cargo test --workspace --exclude spec-tests --no-default-features --features alloc
• cargo check -p libssz -p libssz-types -p libssz-merkle --target thumbv7m-none-eabi --no-default-features --features alloc

[MegaRedHand]

This is the equivalent of ssz_decode_fixed_vec but in the derive macro