fix(Azure): guard missing description for VectorStores (#765)

Azure OpenAI's vector-store endpoint does not yet return the `description`
field that was added in v0.19.1, causing "Undefined array key description"
errors. Default to null when the key is absent, consistent with how other
optional fields are handled across the codebase.

Co-authored-by: jesse-bos <jesse.bos@dij.digital>

Author: jesse-bos

src/Responses/VectorStores/VectorStoreResponse.php

@@ -55,7 +55,7 @@ public static function from(array $attributes, MetaInformation $meta): self
             $attributes['object'],
             $attributes['created_at'],
             $attributes['name'],
-            $attributes['description'],
+            $attributes['description'] ?? null,
             $attributes['usage_bytes'],
             VectorStoreResponseFileCounts::from($attributes['file_counts']),
             $attributes['status'],