docs(router): document @mcpTool directive for custom tool names

[shamashel]

Summary

Documents the new @mcpTool(name: "...") directive that allows users to override default MCP tool names.

Related to: wundergraph/cosmo#2308
Implementation PR: wundergraph/cosmo#2433

Changes

• Added "Custom Tool Names" subsection under "Operation Naming and Tool Generation"
• Includes example showing directive usage
• Documents key behaviors (auto-stripping, fallback to default naming)

Preview

The new section shows:

query GetEmployee($id: Int!) @mcpTool(name: "get_employee_by_id") {
  employee(id: $id) { ... }
}

This creates tool get_employee_by_id instead of default execute_operation_get_employee.

[shamashel]

Closing in favor of #221, which documents the config-based approach (omit_operation_prefix) from wundergraph/cosmo#2441 instead of the directive-based approach.