🧰 fix: Convert const to enum in MCP Schemas for Gemini Compatibility (#11784)

* fix: Convert `const` to `enum` in MCP tool schemas for Gemini/Vertex AI compatibility

  Gemini/Vertex AI rejects the JSON Schema `const` keyword in function declarations
  with a 400 error. Previously, the Zod conversion layer accidentally stripped `const`,
  but after migrating to pass raw JSON schemas directly to providers, the unsupported
  keyword now reaches Gemini verbatim.

  Add `normalizeJsonSchema` to recursively convert `const: X` → `enum: [X]`, which is
  semantically equivalent per the JSON Schema spec and supported by all providers.

* fix: Update secure cookie handling in AuthService to use dynamic secure flag

Replaced the static `secure: isProduction` with a call to `shouldUseSecureCookie()` in the `setOpenIDAuthTokens` function. This change ensures that the secure cookie setting is evaluated at runtime, improving cookie handling in development environments while maintaining security in production.

* refactor: Simplify MCP tool key formatting and remove unused mocks in tests

- Updated MCP test suite to replace static tool key formatting with a dynamic delimiter from Constants, enhancing consistency and maintainability.
- Removed unused mock implementations for `@langchain/core/tools` and `@librechat/agents`, streamlining the test setup.
- Adjusted related test cases to reflect the new tool key format, ensuring all tests remain functional.

* chore: import order

api/server/services/MCP.js

@@ -11,8 +11,9 @@ const {
   MCPOAuthHandler,
   isMCPDomainAllowed,
   normalizeServerName,
-  resolveJsonSchemaRefs,
+  normalizeJsonSchema,
   GenerationJobManager,
+  resolveJsonSchemaRefs,
 } = require('@librechat/api');
 const {
   Time,
@@ -443,7 +444,7 @@ function createToolInstance({
   const { description, parameters } = toolDefinition;
   const isGoogle = _provider === Providers.VERTEXAI || _provider === Providers.GOOGLE;
 
-  let schema = parameters ? resolveJsonSchemaRefs(parameters) : null;
+  let schema = parameters ? normalizeJsonSchema(resolveJsonSchemaRefs(parameters)) : null;
 
   if (!schema || (isGoogle && isEmptyObjectSchema(schema))) {
     schema = {