feat: Add cell metadata compatibility and tests

[Yahiewi]

This PR adds compatibility and round-trip idempotency to the four formats (in particular MyST and Percent by supporting more complex metadata).

Code Changes:

1. Parsing & Serialization Enhancements:

MyST Format (.md):
-Updated parseMystMd.ts to parse code cell metadata using the parseYAMLBlock helper rather than the line-by-line regex-based parseOptions. This fixes reading nested configuration parameters (like nbgrader settings).
-Updated toMystMd.ts to serialize cell metadata conditionally: flat/simple metadata goes to compact colon format (with blank line padding), while nested metadata goes to standard YAML blocks (with no blank line padding).

Percent Format (.py):
-Updated parsePy.ts to handle inline JSON block options and space-separated key-value pairs in cell delimiter lines.
-Updated toPy.ts to extract cell name/title first and write it as a positional argument, then serialize the remaining metadata using JSON or key-value pairs depending on complexity.

2. New helper functions (utils.ts):
   -Added preprocessYamlLines to support parsing multi-line arrays in YAML headers.
   -Added cleanCellMetadata to filter out transient/session-only cell metadata keys (like trusted, collapsed, scrolled, ExecuteTime, execution, etc.) to match Jupytext's serialization filters while fully preserving functional metadata (such as locked and nbgrader).
   -Added parseDoubleQuotedString and escapeNonAsciiYAML to decode and restore PyYAML hex-encoded sequences (preserving special characters like "é" for example).
   -Added pythonStyleJSON to format serialized JSON cell metadata with spaces after colons/commas to mirror Python's default representation.
   -Renamed parseFrontMatter to parseYAMLBlock because its usage is more generic now as it can parse cell YAML blocks.

3. Demo Updates (App.tsx)
   Added VARIABLES_MYST_SAMPLE (the C++17 variables notebook with custom metadata) to the React demo application's samples.

4. Tests:
   Created a new roundtrip.test.ts file to test round-trips across all four plaintext formats.

Screencast:

Screencast.from.2026-06-10.14-28-03.mp4

I still need to figure out an analogue for jupytext.toml files which can have metadata filter configurations.

[github-actions]

🎉 This PR is included in version 1.9.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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