Fix `InvalidArgumentException` when using nested condition groups in Search API queries

When executing a Search API query that contains nested condition groups (e.g., an `AND` group containing another `AND` or `OR` group), the Qdrant provider throws the following error:
`InvalidArgumentException: Qdrant condition must have a "key" field in Drupal\ai_vdb_provider_qdrant\Plugin\VdbProvider\QdrantProvider->validateQdrantCondition()`

This happens because the `validateQdrantCondition()` method strictly expects a flat condition with a `key` property, but it is being passed a nested group structure (e.g., an array starting with `must`, `should`, or `must_not`).

I stumbled on this issue, using the ai relatted content module, which adds a condition to exclude the current node while doing a vector search to get related content.