enhance(oauth): pre-register a builtin OAuth2 application for gitea-vscode (#39785) (#39801)

Backport #39785 by @bircni

The [Gitea VS Code extension](https://gitea.com/gitea/gitea-vscode)
currently signs in through the builtin `tea` OAuth2 application. Gitea
keeps one grant per user and application, so anyone who has already
signed in with the tea CLI gets `a grant exists with different scope`
when the extension requests its own scopes, and sign-in fails
(gitea/gitea-vscode#24).

This adds a dedicated builtin application for the extension, following
the same mechanism as the existing CLI entries:

| | |
|---|---|
| client ID | `75a9ec3e-42b5-40cb-9144-9ea96ecd56a6` |
| config name | `gitea-vscode` |
| display name | `gitea-vscode` |
| redirect URIs | `http://127.0.0.1`, `https://127.0.0.1` (loopback, any
port) |

It is enabled by default via `[oauth2].DEFAULT_APPLICATIONS`. Instances
that set that option explicitly need to add `gitea-vscode` to use it.

The extension will use this client on Gitea >= 28.2.0 and keep falling
back to `tea` on older servers, so this should be backported to v28.

Co-authored-by: bircni <bircni@icloud.com>

models/auth/oauth2.go

@@ -88,6 +88,16 @@ func BuiltinApplications() map[string]*BuiltinOAuth2Application {
 		DisplayName:  "Gitea App",
 		RedirectURIs: []string{"com.gitea.app://oauth/callback"},
 	}
+	m["75a9ec3e-42b5-40cb-9144-9ea96ecd56a6"] = &BuiltinOAuth2Application{
+		ConfigName:  "gitea-vscode",
+		DisplayName: "gitea-vscode",
+		RedirectURIs: []string{
+			"http://127.0.0.1",
+			"https://127.0.0.1",
+			"vscode://gitea.gitea-for-vscode/auth/callback",
+			"vscode-insiders://gitea.gitea-for-vscode/auth/callback",
+		},
+	}
 	return m
 }