🪄 refactor: Simplify MCP Tool Content Formatting to Unified String Output (#12352)

* refactor: Simplify content formatting in MCP service and parser

- Consolidated content handling in `formatToolContent` to return a plain-text string instead of an array for all providers, enhancing clarity and consistency.
- Removed unnecessary checks for content array providers, streamlining the logic for handling text and image artifacts.
- Updated related tests to reflect changes in expected output format, ensuring comprehensive coverage for the new implementation.

* fix: Return empty string for image-only tool responses instead of '(No response)'

When artifacts exist (images/UI resources) but no text content is present,
return an empty string rather than the misleading '(No response)' fallback.
Adds missing test assertions for image-only content and standardizes
length checks to explicit `> 0` comparisons.

api/server/services/MCP.js

@@ -15,13 +15,7 @@ const {
   GenerationJobManager,
   resolveJsonSchemaRefs,
 } = require('@librechat/api');
-const {
-  Time,
-  CacheKeys,
-  Constants,
-  ContentTypes,
-  isAssistantsEndpoint,
-} = require('librechat-data-provider');
+const { Time, CacheKeys, Constants, isAssistantsEndpoint } = require('librechat-data-provider');
 const {
   getOAuthReconnectionManager,
   getMCPServersRegistry,
@@ -605,9 +599,6 @@ function createToolInstance({
       if (isAssistantsEndpoint(provider) && Array.isArray(result)) {
         return result[0];
       }
-      if (isGoogle && Array.isArray(result[0]) && result[0][0]?.type === ContentTypes.TEXT) {
-        return [result[0][0].text, result[1]];
-      }
       return result;
     } catch (error) {
       logger.error(