feat(providers): allow provider_type changes on update with SSRF re-validation

Remove blanket rejection of provider_type on update. Instead, when
provider_type changes, re-validate the existing api_base against the
new type to prevent SSRF via ACP→non-ACP type switch.

This enables provider type changes in onboarding/setup flows while
maintaining security: ACP providers skip URL validation, so switching
from ACP to another type now forces URL re-validation.

internal/http/providers.go

@@ -376,13 +376,6 @@ func (h *ProvidersHandler) handleUpdateProvider(w http.ResponseWriter, r *http.R
 		}
 	}
 
-	// provider_type is immutable after creation — changing it would bypass
-	// security checks (e.g. ACP skips URL validation). Reject if client tries.
-	if _, ok := updates["provider_type"]; ok {
-		writeJSON(w, http.StatusBadRequest, map[string]string{"error": i18n.T(locale, i18n.MsgInvalidRequest, "provider_type cannot be changed after creation")})
-		return
-	}
-
 	// Strip masked API key — don't overwrite real value with "***"
 	if apiKey, ok := updates["api_key"]; ok {
 		if s, _ := apiKey.(string); s == "***" || s == "" {
@@ -415,6 +408,9 @@ func (h *ProvidersHandler) handleUpdateProvider(w http.ResponseWriter, r *http.R
 	if displayName, ok := updates["display_name"].(string); ok {
 		candidate.DisplayName = displayName
 	}
+	if pt, ok := updates["provider_type"].(string); ok && pt != "" {
+		candidate.ProviderType = pt
+	}
 	if settings, ok := updates["settings"]; ok {
 		rawSettings, err := marshalJSONRaw(settings)
 		if err != nil {
@@ -424,6 +420,11 @@ func (h *ProvidersHandler) handleUpdateProvider(w http.ResponseWriter, r *http.R
 		candidate.Settings = rawSettings
 	}
 
+	// Re-validate URLs against the (possibly new) provider type.
+	// When provider_type changes, existing api_base must also pass validation
+	// for the new type — prevents SSRF via ACP→non-ACP type switch.
+	typeChanged := candidate.ProviderType != currentProvider.ProviderType
+
 	if apiBase, ok := updates["api_base"]; ok {
 		if s, _ := apiBase.(string); s != "" {
 			if err := validateProviderURL(s, candidate.ProviderType); err != nil {
@@ -431,6 +432,11 @@ func (h *ProvidersHandler) handleUpdateProvider(w http.ResponseWriter, r *http.R
 				return
 			}
 		}
+	} else if typeChanged && candidate.APIBase != "" {
+		if err := validateProviderURL(candidate.APIBase, candidate.ProviderType); err != nil {
+			writeJSON(w, http.StatusBadRequest, map[string]string{"error": err.Error()})
+			return
+		}
 	}
 	if baseURL, ok := updates["base_url"]; ok {
 		if s, _ := baseURL.(string); s != "" {