Fix attachment cookie handling with Firefox First-Party Isolation

[pellaeon]

This PR:

1. Resolve cookies using the originating tab’s Firefox FPI and container context, and avoid combining cookies from unrelated partitions.

2. Gracefully continue attachment downloads when cookie enumeration fails, scope temporary Cookie headers to the exact request URL, and preserve browser cookie semantics when injecting contextual cookies.

3. Also retain compatibility with older Chromium versions that reject partitionKey and preserve the tab context during bot-bypass retries.

Fixes #226

The code changes are proposed by a LLM and manually reviewed by me line-by-line. Manual testing shows that this PR fixes PDF saving issues on Firefox with FPI enabled.

[adomasven]

Could you provide URLs, procedures and software versions with which you manually tested this and confirmed it works?

[pellaeon]

• PDF URL: https://www.usenix.org/system/files/conference/usenixsecurity26/sec26_prepub_hu.pdf
• Firefox version: 151.0.3 (amd64, Ubuntu 26.04, from https://launchpad.net/~mozillateam/+archive/ubuntu/ppa/+packages )
• Firefox about:config : set privacy.firstparty.isolate to true

Before fix behavior:

1. navigate firefox to PDF URL
2. open Zotero app (flatpak 9.0.4)
3. click the Zotero connector icon to save
4. the icon turns into a red X mark, says "save failed" , PDF did not show up in Zotero app

After fix behavior:
4. the icon turns into a green check mark, PDF shows up in Zotero app.

[adomasven]

Thanks for the PR. This wasn't quite in the state that we'd be able to merge, but I used it as a starting point for a smaller change. It's now available in the Zotero Connector Beta