AzureVectorStore throws UnsupportedOperationException when performing similarity search due to immutable metadata map

[abalosache]

Bug Description

The AzureVectorStore throws an UnsupportedOperationException when performing similarity searches. The error occurs because the code attempts to modify an immutable metadata map during result processing.

Environment

• Spring AI version: 1.0.1
• Java version: 21

Steps to Reproduce

1. Configure AzureVectorStore with Azure AI Search backend
2. Index documents using Azure AI Search indexer with skillset (document chunking and embedding generation)
3. Perform a similarity search using Spring AI's VectorStore interface:

   List<Document> results = vectorStore.similaritySearch("your query here");

[jinlee1703]

I can reproduce this when the Azure result metadata map is immutable. The mapping logic later injects distance into the metadata and triggers UnsupportedOperationException.
I’ll submit a PR to defensively copy incoming metadata into a mutable map before building the Document (and then set DocumentMetadata.DISTANCE). This mirrors the defensive fixes we’ve applied in other stores.

[injae-kim]

Fix PR: #4131 PTAL :)