fix: respect ByteStream encoding in HTMLToDocument and MarkdownToDocument

[greymoth-jp]

Summary

HTMLToDocument and MarkdownToDocument both hardcode .decode("utf-8"), which raises UnicodeDecodeError (or silently corrupts output) for any non-UTF-8 ByteStream whose encoding is passed via ByteStream.meta["encoding"]. TextFileToDocument already handles this correctly via an encoding constructor param and a bytestream.meta.get("encoding", self.encoding) fallback.

This PR brings both converters into line with TextFileToDocument:

• Add an encoding: str = "utf-8" parameter to HTMLToDocument.__init__ and MarkdownToDocument.__init__
• Change the hardcoded .decode("utf-8") calls to bytestream.meta.get("encoding", self.encoding) in the run() method of each converter
• Include encoding in HTMLToDocument.to_dict() so the param survives serialization round-trips (.from_dict() already routes through default_from_dict which handles it automatically)
• Add unit tests (test_bytestream_encoding_from_meta and test_bytestream_encoding_from_init) for both converters using a latin-1 ByteStream

Reproduction (before fix):

from haystack.components.converters import HTMLToDocument
from haystack.dataclasses import ByteStream

bs = ByteStream(b"<p>caf\xe9</p>", meta={"encoding": "latin-1"})
HTMLToDocument().run(sources=[bs])
# UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9

This PR was fully generated with an AI assistant. I have reviewed the changes and run the relevant tests (29 passed, 0 failed).

Test plan

• pytest test/components/converters/test_html_to_document.py test/components/converters/test_markdown_to_document.py -k "not integration" — 29 passed
• New tests test_bytestream_encoding_from_meta and test_bytestream_encoding_from_init added for both HTMLToDocument and MarkdownToDocument
• Existing test_serde updated to assert encoding round-trips through to_dict / from_dict

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