The mcpConfigs collection currently stores both _id (a MongoDB ObjectId) and configId (a UUID string). Unify these by storing the UUID directly as _id, eliminating the redundant field and its index (idx_configId).

Requires:

• Update CreateMcpConfigsCollectionMigration.java to remove configId from the JSON schema validator
• Update McpConfigRepository to query/filter on _id instead of configId