REST: Add pagination support for list_namespaces

[vishnuprakaz]

Follows the REST catalog spec:

• /v1/{prefix}/namespaces: https://github.com/apache/iceberg/blob/main/open-api/rest-catalog-open-api.yaml
• PageToken: https://github.com/apache/iceberg/blob/e7a5a87f26f9de5b200254155aa037368b13a29c/open-api/rest-catalog-open-api.yaml#L2233-L2256

Rationale for this change

RestCatalog.list_namespaces was making a single HTTP request and returning only the first page of results. If the server paginates the response, PyIceberg silently returned an incomplete namespace list.

This is the same gap that was fixed for list_views by @ebyhr in #3349 also is being fixed for list_tables in #3348.

Are these changes tested?

Yes, includes unit tests for paginated cases:

• test_list_namespaces_paginated_200 — verifies all pages are fetched when next-page-token is omitted on the last page
• test_list_namespaces_paginated_200_none_next_page_token — verifies all pages are fetched when next-page-token is explicitly null on the last page

Are there any user-facing changes?

• Before: returned incomplete namespace list when server paginated (only first page)
• After: returns complete namespace list (fetches all pages)

Closes #3382