fix: resolve in-place dataclass mutations

[Anandesh-Sharma]

Summary

• Replaces in-place dataclass attribute mutations (doc.score = x, stream.mime_type = y, etc.) with dataclasses.replace() across 12 source files
• Resolves the deprecation warnings introduced by the dataclass mutation detection added in feat: Warn on in-place mutation of dataclass instances #10650
• Adds a release note documenting the change

Related Issue

Fixes #10659

Changes by file

File	Mutation fixed
chat_prompt_builder.py	rendered_message._content = [...]
pipeline.py	pipeline_snapshot.agent_snapshot = ..., pipeline_snapshot.break_point = ...
file_to_image.py	bytestream.mime_type = ...
llm_metadata_extractor.py	doc_copy.content = content
link_content.py	stream.mime_type = ... (3 places: sync run, sync multi-url, async)
document_joiner.py	doc.score = ... (3 methods: _merge, _reciprocal_rank_fusion, _distribution_based_rank_fusion)
hugging_face_tei.py	doc_copy.score = ...
meta_field.py	document.score = ...
sentence_transformers_similarity.py	document.score = score
transformers_similarity.py	deduplicated_documents[i].score = ...
extractive.py	answer.query = ..., answer.meta = {}, answer.meta.update(...)
in_memory/document_store.py	doc.embedding = None

Test plan

• Existing unit tests should pass (this is a mechanical refactoring that preserves behavior)
• Verify no new dataclass mutation warnings are emitted in the changed code paths

🤖 Generated with Claude Code

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[anakin87]

@Anandesh-Sharma could you please sign the CLA?

[anakin87]

Superseded by #10702