Add entity name overlap tests

Following up on the discussion in #4148 (related to #4108), it seems valuable to add generic entity name collision tests to `polaris-tests`.

Previous work on this from @Subham-KRLX (class `PolarisRestCatalogIntegrationBase`):
```
@Test
public void testCreateTableNameThatCollidesWithNamespace() {
Assumptions.assumeThat(testNamespaceTableNameCollision()).isTrue();

Namespace parentNamespace = Namespace.of("ns1");
restCatalog.createNamespace(parentNamespace);
restCatalog.createNamespace(Namespace.of("ns1", "clash"));

assertThatThrownBy(
() ->
restCatalog
.buildTable(TableIdentifier.of(parentNamespace, "clash"), SCHEMA)
.create())
.isInstanceOf(AlreadyExistsException.class)
.hasMessageContaining("already exists");
}
```

Each name within a namespaces should only map to at most one entity type (Table, (nested) Namespace, View, etc.)

Start with PolarisRestCatalogIntegrationBase and the existing testCreateTableNameThatCollidesWithNamespace example in polaris-tests. Add generic collision coverage for Table, nested Namespace, View, and other named entities; done when a name cannot map to more than one entity type and conflicting creation reports AlreadyExistsException.

Written by the indexing model from the issue text.