Exporter caches are never reset, so bulk export writes stubs into the second and later files

Problem/Motivation

ContentExporter::$entityReferenceCache and $entityOutputCache are instance properties that doExportToYml() never clears, so they leak between exports in the same request.

Several export paths emit only a stub when the reference cache reports an entity as already seen. It is correct within one document and wrong across documents: each .yml is imported on its own, and ContentImporter::createStubEntity() then saves a hollow entity with no custom field values, and without an error.

Bulk export calls doExportToYml() once per entity in a loop (ContentFileGenerator::generateBulkZipFile()), so every file after the first can be short.

Steps to reproduce

Bulk export two entities that share a referenced entity. The second file will be smaller than the same entity exported on its own as it only contains a stub for the shared entity.

Proposed resolution

Clear both caches at the top of doExportToYml() so that every document is self-contained.

User interface changes

None.

API changes

None.

Data model changes

None.