refactor(detect): reduce complexity of parse_header_line in detect.rs

[github-actions]

Summary

parse_header_line in src/detect.rs contained an inline quoted-string parser directly inside the source= match arm, adding ~4 extra levels of nesting (while loop → if-let strip_prefix → if-let strip_prefix quote → while-let char scan → if escape/quote/else).

What changed

Extracted a private parse_header_value(input: &str) -> (String, &str) helper that handles both:

• Quoted values — scans char-by-char with backslash-escape support, returning the unescaped value and the remaining tail after the closing "
• Unquoted values — takes up to the next space

parse_header_line is now a flat dispatch loop: strip prefix → call parse_header_value or take-until-space → advance remaining. Maximum nesting depth drops from ~7 to ~3.

Verification

• All existing parse_header_line tests pass (test_parse_header_line_*)
• Full cargo test green — no regressions
• cargo clippy --all-targets --all-features — clean

Complexity

	Before	After
Max nesting depth	~7	~3
Function length	59 lines	22 lines (+ 22-line helper)

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