Fixed process_metadata function that would raise RuntimeError: dictionary changed size during iteration when metadata contained any of the excluded keys (content, pages, tables, paragraphs, sections, figures).

The function was deleting keys while iterating over dict.items(), which invalidates the iterator in Python 3. Now builds a new dict instead of mutating the original.

Why this has never been reported

This bug has never surfaced in practice due to how the code paths are structured:

1. Multitenancy mode users - The multitenancy Milvus client (and others) doesn't call process_metadata at all, passing metadata directly without processing.

2. Standard mode users - Before metadata reaches process_metadata, it passes through filter_metadata first (in process_file), which already removes the excluded keys. By the time process_metadata is called, those keys are already gone.

3. Document loaders - The custom loaders (MinerU, Mistral OCR, Datalab Marker, Docling, Tika) don't include these keys in their metadata output. Only certain LangChain loaders like AzureAIDocumentIntelligenceLoader might return them, but they get filtered upstream.

Why it should still be fixed

Despite being unreachable in current code paths, this is a latent bug that could cause failures if:

• A new code path calls process_metadata directly without prior filtering
• A new document loader returns metadata with these keys
• Someone refactors the existing flow and removes the upstream filter_metadata call

Fixing it ensures the function works correctly in isolation, as its signature and docstring imply it should.

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