fix(rust): handle single-character ALPN according to spec (#325)

[Aditya-9-6]

Summary

According to the JA4 specification, a single-character ALPN value should use that character for both the first and last positions (e.g., "x" -> "xx" instead of "x0").

This aligns the Rust implementation with the Python fix implemented in #322.

Changes

• In rust/ja4/src/tls.rs (first_last), when chars.next_back() returns None for single-character strings, fall back to first using .or(first).
• Updated test_first_last() and test_first_last_non_ascii() to verify single-character ASCII and non-ASCII ALPN behavior.
• Added test_single_char_alpn_fingerprints() to verify that both client (PartsOfClientFingerprint) and server (ServerStats) JA4 outputs format single-character ALPN as "xx".
• Added an entry in rust/CHANGELOG.md under [Unreleased].

Verification

Local test verification passed with zero failures or warnings:

• cargo fmt --check --all (Clean)
• cargo clippy --all-features --workspace -- -D warnings (Clean, 0 warnings)
• cargo test --locked --all-features --all-targets --workspace (All 25 tests passed)
• cargo test --locked --all-features --doc --workspace (Passed)
• cargo doc --no-deps --all-features --document-private-items --workspace (Passed)

Closes #325

[vlvkobal]

I removed the changelog entry since rust/CHANGELOG.md hasn’t been maintained for quite a while. I think we should probably remove the stale changelog rather than start updating it selectively again here.