fix: preserve metadata key named "meta" in Document.from_dict()

[Otis0408]

What

Document.to_dict() defaults to flatten=True, merging every metadata key up into the top-level dict. Document.from_dict() reverses this by popping the top-level "meta" key back out as the meta constructor parameter. This breaks when a document's metadata legitimately contains a JSON-serializable key literally named "meta": from_dict() pops that value as the meta parameter and splats it with {**meta}. When the value is not a mapping this raises TypeError: 'str' object is not a mapping, so a normal document cannot be serialized and read back through the default round-trip.

Fix

In from_dict(), after popping the "meta" key, check its type. A value that is not a mapping cannot be a valid meta parameter (which must be a dictionary); it can only be a flattened metadata key named "meta". Treat it as flattened metadata instead of splatting it, so the value is preserved and the round-trip succeeds. A dict value under "meta" continues to be treated as the meta parameter, preserving existing behavior (test_from_dict_with_parameters, test_from_dict_with_flat_and_non_flat_meta).

Before / after

from haystack.dataclasses import Document

doc = Document(content="hi", meta={"meta": "value"})

# Before: TypeError: 'str' object is not a mapping
# After:  round-trips cleanly
restored = Document.from_dict(doc.to_dict())
assert restored == doc
assert restored.meta == {"meta": "value"}

Tests

Added test_to_dict_from_dict_roundtrip_with_meta_key_named_meta. It fails on the pre-fix source with the TypeError above and passes with the fix. The full test/dataclasses/test_document.py suite passes (30 tests).

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/dataclasses
document.py
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