Open-API: RESTCatalogServer.stop() leaks the backend catalog

**Apache Iceberg version**
main @ 3038fde68

**Query engine**
N/A — engine-agnostic test fixture

**Please describe the bug**
`RESTCatalogServer.stop()` only stops the Jetty `httpServer`; the backend `Catalog` created in `initializeBackendCatalog()` (a `Closeable` `JdbcCatalog` by default, holding a JDBC connection pool and `FileIO`) is never closed.
Offending code: `RESTCatalogServer.stop()` (`open-api/src/testFixtures/java/org/apache/iceberg/rest/RESTCatalogServer.java` line 137-141).
`RESTCatalogAdapter.close()` (`core/src/test/java/org/apache/iceberg/rest/RESTCatalogAdapter.java` line 680-685) documents that "the calling test is responsible for closing the underlying catalog" — the server is that owner but does not do it.

**Steps to reproduce**
1. Call `new RESTCatalogServer(config).start(false)`.
2. Call `server.stop()`.
3. The backend `JdbcCatalog`'s JDBC connection pool and `FileIO` are still open.

Expected: backend catalog is closed along with the server.
Actual: backend catalog resources leak.

`RESTServerExtension` (used as a class-level JUnit extension in the REST Compatibility Kit suite and in Spark 3.5/4.0/4.1 `TestBaseWithCatalog`) triggers this on every test class teardown.

**Additional context**
N/A.

Start in open-api/src/testFixtures/java/org/apache/iceberg/rest/RESTCatalogServer.java, reading initializeBackendCatalog() and stop() to trace ownership of the backend Catalog. Check core/src/test/java/org/apache/iceberg/rest/RESTCatalogAdapter.java for the closing contract, then run the REST Compatibility Kit tests; done means server shutdown also closes the backend catalog and its resources.

Written by the indexing model from the issue text.