Improve HTTP header parsing to handle quoted strings with delimiters

[coderabbitai]

Problem

The current HTTP header parsing implementation in the codebase uses naive string splitting methods that fail when quoted values contain delimiter characters. This affects the ForwardedHeader implementation and potentially other header parsers.

Example Issue

A header value like:

for="[2001:db8:cafe::17]:4711";desc="semi;colon"

Will be incorrectly parsed because the simple split(';') and split('=') methods don't respect quoted strings and escapes.

Current Implementation

The parsing uses:

final pairStrings = elementStr.split(';');
final parts = pairStr.split('=');

Suggested Solutions

1. Reuse HeaderValue.parse from package:http_parser <-- no such thing
2. Implement a custom state machine that properly handles quoted strings and escapes
3. Use a proper HTTP header parser that respects RFC standards

Impact

This is a broader architectural issue that could affect multiple header types beyond just the Forwarded header.

References

• Original discussion: feat: Typed forwarded header #101 (comment)
• Related PR: feat: Typed forwarded header #101

[nielsenko]

There is no such thing as HeaderValue.parse exported from the http_parser (@coderabbitai is hallucinating), but we may steal the private scan.dart file and build on that.

[SandPod]

Relates to #113