Front matter: a date nested in a dict or list crashes the build (TypeError in dict_to_fm_field_list)

[PiotrRozenfeld]

What version of myst-parser are you using?

5.1.0

What version dependencies are you using?

Sphinx 8.2.3, docutils 0.21.2, Python 3.12

What operating system are you using?

Windows

Describe the Bug

A YAML date nested inside a dict or a list in the front matter stops the whole Sphinx build:

File "myst_parser/mdit_to_docutils/base.py", line 1390, in dict_to_fm_field_list
  value = json.dumps(value)
TypeError: Object of type date is not JSON serializable

dict_to_fm_field_list lets top-level date / datetime values through the isinstance check
(this was the fix for #252), but a date one level down reaches json.dumps(value) with no
default=. The except (ValueError, RecursionError) around that call does not catch TypeError,
so this is an exception, not a warning, and --keep-going does not help.

Expected Behavior

The build succeeds. Either of these one-line changes fits the existing code:

• json.dumps(value, default=str) - the nested date becomes an ISO string, the same as a
  top-level date after str(value).
• Add TypeError to the existing except (ValueError, RecursionError), so the field is skipped
  with the same MD_TOPMATTER warning.

I tested the first one on master (Linux and Windows): all four cases below then build with no
warnings.

To Reproduce

conf.py:

extensions = ["myst_parser"]

index.md:

---
review:
  date: 2026-08-03
---
# Test

Run sphinx-build -b html . _build. The build stops with the TypeError above.

Results for four variants (same result on 5.1.0 and on master, on Linux and on Windows):

Front matter	Result
last_updated: 2026-08-03 (top level)	builds
review: {date: 2026-08-03} (nested in a dict)	TypeError
reviewed: [2026-08-03] (nested in a list)	TypeError
review: {date: "2026-08-03"} (nested, quoted)	builds