Put MCP Server access behind beta features setting

What does this MR do and why?

This MR puts MCP Server access behind the instance beta features setting.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Generate a new PAT via the UI.
2. In rails console, set the token scopes to ["mcp"].

   PersonalAccessToken.first.update(scopes: ["mcp"])

3. In rails console, enable the feature flags

   Feature.enable(:mcp_server)
   Feature.enable(:mcp_server_new_implementation)

4. In rails console, ensure the AI Features Beta setting is enabled.

   ApplicationSetting.first.update(instance_level_ai_beta_features_enabled: true)

5. Verify the API request to MCP server is successful.

   curl --request POST \
     --url http://gdk.test:8080/api/v4/mcp \
     --header "Authorization: Bearer $GITLAB_TOKEN" \
     --header 'Content-Type: application/json' \
     --data '{
     "jsonrpc": 2.0,
     "method": "tools/call",
     "id": 1,
     "params": {
       "name":"get_mcp_server_version"
     }
   }
   '

6. In rails console, ensure the AI Features Beta setting is disabled.

   ApplicationSetting.first.update(instance_level_ai_beta_features_enabled: false)

7. Verify the same API request to MCP server returns 404 Not Found.

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Related to #568129 (closed)