fix: handle digest quoted strings correctly

[ryanfowler]

Summary

• preserve UTF-8 while parsing Digest quoted strings
• escape backslashes and quotes in generated authorization parameters
• reject malformed or unterminated quoted strings
• add coverage for UTF-8 and quoted-pair escaping

Validation

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins