fix(search): expose markdown frontmatter on chunks

[RerankerGuo]

Summary

• copy parsed Markdown frontmatter onto emitted FileChunk metadata
• preserve JSON-safe frontmatter values such as conversation_date for search filters
• add coverage showing chunk metadata and metadata search_filter plumbing

Refs #302

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[ployts]

Thanks for working on this! Exposing the markdown frontmatter on chunks is a great addition for search filtering.
To ensure backward compatibility and prevent any unexpected side effects for existing users, I suggest adding a configuration flag (e.g., enable_frontmatter_metadata or include_frontmatter_in_metadata) to toggle this behavior. Could we set it to False by default?
This would allow users to opt-in when needed while keeping the default behavior unchanged. Let me know your thoughts!

[RerankerGuo]

Updated per feedback. I added the include_frontmatter_in_metadata flag, set the default config value to false, and adjusted tests to cover both the backward-compatible default and the opt-in metadata behavior.

Verification:

• Targeted pytest: 2 passed
• Pre-commit on touched files: passed