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>

modules/setting/oauth2.go

@@ -108,7 +108,7 @@ var OAuth2 = struct {
 	JWTSigningAlgorithm:        "RS256",
 	JWTSigningPrivateKeyFile:   "jwt/private.pem",
 	MaxTokenLength:             math.MaxInt16,
-	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"},
+	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app", "gitea-vscode"},
 }
 
 func loadOAuth2From(rootCfg ConfigProvider) {

modules/setting/oauth2_test.go

@@ -62,7 +62,7 @@ func TestGetGeneralSigningSecretSave(t *testing.T) {
 func TestOauth2DefaultApplications(t *testing.T) {
 	cfg, _ := NewConfigProviderFromData(``)
 	loadOAuth2From(cfg)
-	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"}, OAuth2.DefaultApplications)
+	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app", "gitea-vscode"}, OAuth2.DefaultApplications)
 
 	cfg, _ = NewConfigProviderFromData(`[oauth2]
 DEFAULT_APPLICATIONS = tea