fix: normalize Cookie header when proxying (RFC 7540)

[jakst]

This fixes an HTTP/2 -> backend proxying bug where multiple Cookie header fields (or comma-joined cookie values) could be forwarded in an invalid form, causing the backend to miss cookies or parse them incorrectly. Per RFC 7540 §8.1.2.5, multiple Cookie header fields must be concatenated using semicolon when passed into a non-HTTP/2 context.

Implementation

• Add linkup::normalize_cookie_header(...) to combine multiple Cookie headers with ; .
• Also repairs the common bad case where Cookie fields were previously joined with commas (treat commas as split points, then re-join with semicolon).
• Apply normalization in both proxy paths:
  • local-server/src/lib.rs (HTTP proxy + WS handshake header copy)
  • worker/src/lib.rs

Notes
My Rust is a bit rusty, and I’m not familiar with the Linkup repo, so these changes were heavily LLM-assisted. I’ve reviewed the logic and it looks sound, and I verified locally that a build with these changes resolves the issues we're having in the Endform repo.

[augustoccesar]

Thank you for the fix! ⭐
One question and a couple of suggestions.

[jakst]

Thank you for the fix! ⭐ One question and a couple of suggestions.

Thanks for the review! All suggestions implemented :)

[augustoccesar]

There seems to be some issues with the worker that are unrelated to this PR.
I will take a look into it.

[augustoccesar]

There seems to be some issues with the worker that are unrelated to this PR. I will take a look into it.

Hopefully #252 fixed this issue.
@jakst Can you rebase your branch?

[augustoccesar]

⭐