refactor(dpi/ftp): move owned Vec<u8> into String, drop redundant copy

[obchain]

Summary

analyze_ftp (src/network/dpi/ftp.rs) built the FTP command string from an already-owned Vec<u8> via:

let command = std::str::from_utf8(&upper).ok()?.to_string();

std::str::from_utf8 borrows the Vec's bytes and returns a &str; .to_string() then allocates a fresh String and copies those bytes in. upper is dropped on the next statement, so the copy is purely shape-driven — nothing else reads the Vec.

first_token_upper (line 169) already returns early with Vec::new() unless every byte in the token passes b.is_ascii_alphabetic(), so the bytes are valid UTF-8 by construction. Take the Vec by value with String::from_utf8 to skip the redundant copy:

let command = String::from_utf8(upper).ok()?;

The ? stays as a defensive guard against a future widening of first_token_upper's accepted byte range, and the new comment records the invariant the guard rests on.

Behavior is unchanged.

Closes #326.

Verification

• cargo test --lib network::dpi::ftp: 13 passed, 0 failed.
• cargo test --lib: all pass.
• cargo clippy --all-targets -- -D warnings: clean.
• cargo fmt --check: clean.

Notes

Fits the recent refactor(dpi/...) allocation-cleanup series (#290, #301, #307, #317) — single-line change, no behavioural impact, removes one String allocation + memcpy per FTP command request packet.