Disable local users for a smooth single-idp mode (#5226)

Add LocalAuthDisabled option to embedded IdP configuration

This adds the ability to disable local (email/password) authentication when using the embedded Dex identity provider. When disabled, users can only authenticate via external
identity providers (Google, OIDC, etc.).

This simplifies user login when there is only one external IdP configured. The login page will redirect directly to the IdP login page.

Key changes:

Added LocalAuthDisabled field to EmbeddedIdPConfig
Added methods to check and toggle local auth: IsLocalAuthEnabled, HasNonLocalConnectors, DisableLocalAuth, EnableLocalAuth
Validation prevents disabling local auth if no external connectors are configured
Existing local users are preserved when disabled and can login again when re-enabled
Operations are idempotent (disabling already disabled is a no-op)

shared/management/http/api/openapi.yml

@@ -294,6 +294,11 @@ components:
           type: boolean
           readOnly: true
           example: false
+        local_auth_disabled:
+          description: Indicates whether local (email/password) authentication is disabled. When true, users can only authenticate via external identity providers. This is a read-only field.
+          type: boolean
+          readOnly: true
+          example: false
       required:
         - peer_login_expiration_enabled
         - peer_login_expiration

shared/management/http/api/types.gen.go

@@ -415,6 +415,9 @@ type AccountSettings struct {
 	// LazyConnectionEnabled Enables or disables experimental lazy connection
 	LazyConnectionEnabled *bool `json:"lazy_connection_enabled,omitempty"`
 
+	// LocalAuthDisabled Indicates whether local (email/password) authentication is disabled. When true, users can only authenticate via external identity providers. This is a read-only field.
+	LocalAuthDisabled *bool `json:"local_auth_disabled,omitempty"`
+
 	// NetworkRange Allows to define a custom network range for the account in CIDR format
 	NetworkRange *string `json:"network_range,omitempty"`