fix(auth): set User-Agent on avatar fetch and sync avatar on link-account register (#37564) (#37588)

## Summary

Fixes
[go-gitea/gitea#37564](https://github.com/go-gitea/gitea/issues/37564):
when an OIDC provider returns a `picture` claim, Gitea is supposed to
download that image as the user's avatar (if `[oauth2_client]
UPDATE_AVATAR = true`). Two latent bugs prevented this from working
consistently:

1. **Default Go User-Agent rejected by some image hosts.**
`oauth2UpdateAvatarIfNeed` used `http.Get`, which sends `User-Agent:
Go-http-client/1.1`. Hosts like `upload.wikimedia.org` reject that UA
with `403`, and every error path silently returned, so the user was left
with an identicon and **no log line** to diagnose the issue.
2. **Link-account *register* path skipped avatar sync.** First-time OIDC
sign-ins where auto-registration is disabled (or required a
username/password retype) go through `LinkAccountPostRegister`, which
created the user but never called `oauth2SignInSync`. So the avatar /
full name / SSH keys from the IdP were dropped on the floor for those
users, even though the existing-account-link path (`oauth2LinkAccount`)
and the auto-register path (`handleOAuth2SignIn`) both already did the
sync.

## Changes

- `routers/web/auth/oauth.go` — `oauth2UpdateAvatarIfNeed` now uses
`http.NewRequest` + `http.DefaultClient.Do`, sets `User-Agent: Gitea
<version>`, and logs every failure path at `Warn` (invalid URL, fetch
error, non-200, body read error, oversize body, upload error). No silent
failures.
- `routers/web/auth/linkaccount.go` — `LinkAccountPostRegister` now
calls `oauth2SignInSync` after a successful user creation, mirroring the
auto-register and link-existing-account flows.
- `tests/integration/oauth_avatar_test.go` — new
`TestOAuth2AvatarFromPicture` integration test with five sub-cases:
- `AutoRegister_FetchesAvatarFromPictureWithGiteaUA` — happy path,
asserts `use_custom_avatar=true`, an avatar hash is set, exactly one
HTTP request was made, and the request carried a `Gitea ` UA. The mock
server enforces the UA prefix to mirror real-world hosts that reject
Go's default UA.
- `AutoRegister_NonOK_DoesNotUpdateAvatar` — server returns 403; user's
avatar must remain unset.
- `AutoRegister_EmptyPicture_NoFetch` — empty `picture` claim must not
trigger any HTTP request.
- `AutoRegister_UpdateAvatarFalse_NoFetch` — `UPDATE_AVATAR=false` must
not trigger any HTTP request.
- `LinkAccountRegister_FetchesAvatarFromPicture` — guards the
`linkaccount.go` fix; without the new `oauth2SignInSync` call this
assertion fails.

## Related

- Upstream issue: go-gitea/gitea#37564
--------------------------------------------

AI Editor was used in this PR

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>

tests/integration/oauth_test.go

@@ -7,6 +7,8 @@ import (
 	"bytes"
 	"encoding/base64"
 	"fmt"
+	"image"
+	"image/png"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -1062,6 +1064,13 @@ func createOAuth2MockProvider() *httptest.Server {
 	var mockServer *httptest.Server
 	mockServer = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {
+		case "/avatar.png":
+			if !strings.HasPrefix(r.Header.Get("User-Agent"), "Gitea ") {
+				http.Error(w, "user agent doesn't match", http.StatusForbidden)
+				return
+			}
+			w.Header().Set("Content-Type", "image/png")
+			_ = png.Encode(w, image.NewRGBA(image.Rect(0, 0, 8, 8)))
 		case "/.well-known/openid-configuration":
 			_, _ = w.Write([]byte(`{
 				"issuer": "` + mockServer.URL + `",