Purpose

Follow up on #9174 by adding display-only schema history APIs to RESTCatalog.

The APIs follow the existing snapshot API pattern and allow clients to inspect complete historical TableSchema metadata through the REST service. Existing table operations and schema-management internals remain unchanged.

API

Catalog

Optional<TableSchema> loadSchema(Identifier identifier, String version);

PagedList<TableSchema> listSchemasPaged(
        Identifier identifier, Integer maxResults, String pageToken);

loadSchema supports:

• LATEST
• EARLIEST
• a schema ID

listSchemasPaged returns schemas in descending schema ID order.

REST protocol

GET /v1/{prefix}/databases/{database}/tables/{table}/schemas/{version}
GET /v1/{prefix}/databases/{database}/tables/{table}/schemas?maxResults=<int>&pageToken=<token>

The responses contain complete TableSchema objects so that schema metadata is preserved.

Backward compatibility

• The new endpoints are used only for schema display.
• Existing getTable, alterTable, and rollbackSchema behavior continues to use the existing implementation.
• No schema capability flag, catalog-backed SchemaManager, or CatalogEnvironment wiring is introduced.
• New clients remain compatible with older REST servers that do not implement the schema endpoints.

Tests

Added coverage for:

• loading LATEST, EARLIEST, and a specific schema ID
• missing and invalid schema versions
• paginated schema listing and ordering
• missing-table error mapping
• compatibility with a server that does not provide the new endpoints, including verification that existing get/alter/rollback operations never call them

Validation performed:

mvn spotless:apply
mvn -pl paimon-api,paimon-core -DskipTests compile
mvn -pl paimon-core -am -Pfast-build -DwildcardSuites=none \
  -DfailIfNoTests=false \
  '-Dtest=MockRESTCatalogTest#testLoadSchema+testListSchemasPaged+testSchemaMethodsTableNotExist+testCompatibilityWithServerWithoutSchemaEndpoints' \
  test

All targeted tests passed: 4 tests, 0 failures, 0 errors.