Make idempotency filter a dedicated module

### Is your feature request related to a problem? Please describe.

_No response_

### Describe the solution you'd like

To address https://github.com/apache/polaris/pull/3803#discussion_r2842138731: create a separate module for the idempotency HTTP filter and related idempotency service classes, instead of bundling them inside runtime/service. The Polaris server (runtime/server) would include this new module so idempotency still works there, but downstream projects that don’t want idempotency could exclude the module and avoid the extra runtime overhead and schema/table management.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by inspecting the idempotency HTTP filter and related service classes currently under runtime/service, then review how runtime/server includes them. Define the new module boundaries and verify that the server retains idempotency while downstream projects can exclude the module and its schema/table management.

Written by the indexing model from the issue text.