2026.8.26-canary.914

fix(server): send User-Agent on GitHub OAuth fetches (#15524)

## Description

Self-hosted GitHub OAuth login fails at token exchange because the
outbound request to `https://github.com/login/oauth/access_token` has no
`User-Agent` header. GitHub then returns 403 ("Request forbidden by
administrative rules"), which is surfaced as
`INVALID_OAUTH_CALLBACK_CODE`.

OAuth `safeFetch` only forwarded `authorization`, `content-type`, and
`accept`, so even a User-Agent on the request would be stripped. This
change:

- allows `user-agent` in OAuth `fetchOptions()`
- always sends `User-Agent: AFFiNE-Server` from `fetchJson()` (covers
token exchange and `api.github.com` user/email fetches)

Fixes #15521

## Checklist

- [x] The PR targets the `canary` branch and its title follows
Conventional Commits
- [x] Tests are added or updated where it makes sense

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved OAuth request compatibility by including a standard
`User-Agent` header.
* Ensured the header is permitted consistently during OAuth token
exchanges.

* **Tests**
* Added coverage to verify case-insensitive handling of the `User-Agent`
header in GitHub OAuth requests.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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