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 -->

---------

packages/backend/server/src/__tests__/oauth/github-user-agent.spec.ts

@@ -0,0 +1,95 @@
+import serverNativeModule from '@affine/server-native';
+import ava, { TestFn } from 'ava';
+import Sinon from 'sinon';
+
+type NativeSafeFetchRequest = {
+  url: string;
+  headers?: Record<string, string>;
+  allowedHeaders?: string[];
+};
+
+const test = ava.serial as TestFn<{
+  requests: NativeSafeFetchRequest[];
+}>;
+
+function headerValue(
+  headers: Record<string, string> | undefined,
+  name: string
+) {
+  const target = name.toLowerCase();
+  for (const [key, value] of Object.entries(headers ?? {})) {
+    if (key.toLowerCase() === target) {
+      return value;
+    }
+  }
+  return undefined;
+}
+
+test.before(t => {
+  const requests: NativeSafeFetchRequest[] = [];
+
+  Sinon.stub(serverNativeModule, 'safeFetch').callsFake(async request => {
+    const nativeRequest = request as NativeSafeFetchRequest;
+    requests.push(nativeRequest);
+    return {
+      status: 200,
+      finalUrl: nativeRequest.url,
+      headers: { 'content-type': 'application/json' },
+      body: Buffer.from(
+        JSON.stringify({
+          access_token: 'github-access-token',
+          scope: 'read:user user:email',
+          token_type: 'bearer',
+        })
+      ),
+    };
+  });
+
+  t.context.requests = requests;
+});
+
+test.after.always(() => {
+  Sinon.restore();
+});
+
+test('github oauth token exchange should send a User-Agent header', async t => {
+  const { OAuthProviderName } = await import('../../plugins/oauth/config');
+  const { OAuthProvider } = await import('../../plugins/oauth/providers/def');
+
+  class Probe extends OAuthProvider {
+    override provider = OAuthProviderName.GitHub;
+
+    getAuthUrl() {
+      return '';
+    }
+
+    async getToken() {
+      return { accessToken: 'token' };
+    }
+
+    async getUser() {
+      return { id: 'id', email: 'user@example.com' };
+    }
+
+    exchange() {
+      return this.postFormJson(
+        'https://github.com/login/oauth/access_token',
+        'code=oauth-code'
+      );
+    }
+  }
+
+  const { requests } = t.context;
+  requests.length = 0;
+
+  await new Probe().exchange();
+
+  t.is(requests.length, 1);
+  t.is(requests[0].url, 'https://github.com/login/oauth/access_token');
+  t.truthy(headerValue(requests[0].headers, 'user-agent'));
+  t.true(
+    (requests[0].allowedHeaders ?? []).some(
+      header => header.toLowerCase() === 'user-agent'
+    )
+  );
+});

packages/backend/server/src/plugins/oauth/providers/def.ts

@@ -85,7 +85,7 @@ export abstract class OAuthProvider {
       timeoutMs: 10_000,
       maxRedirects: 3,
       maxBytes: 1024 * 1024,
-      allowedHeaders: ['authorization', 'content-type', 'accept'],
+      allowedHeaders: ['authorization', 'content-type', 'accept', 'user-agent'],
     };
   }
 
@@ -96,7 +96,14 @@ export abstract class OAuthProvider {
   ) {
     const response = await safeFetch(
       url,
-      { ...init, headers: { ...init?.headers, Accept: 'application/json' } },
+      {
+        ...init,
+        headers: {
+          ...init?.headers,
+          Accept: 'application/json',
+          'User-Agent': 'AFFiNE-Server',
+        },
+      },
       this.fetchOptions(url)
     );