fix unqualified Vec paths in generated code for no_std

[fabracht]

Summary

• Use ::bebytes::Vec instead of unqualified Vec in all generated code
• Fixes 14 locations across lib.rs and structs.rs where Vec::new(), Vec::from(), or return types used unqualified paths
• Update no_std.rs test to import Vec from bebytes instead of alloc, ensuring future regressions are caught

Test plan

• cargo test passes
• cargo test --no-default-features -p bebytes no_std passes
• cargo clippy clean