fix(critical): resolve three critical runtime bugs in MetadataFigure

[lkdmc]

Summary

Three critical bugs that can crash the build or silently produce wrong output.

1. has_caption — NameError when a figure node has no children (__init__.py:779)

has_caption = False was initialised inside the inner for child in node.children: loop body, so it was reset on every iteration and left undefined when node.children is empty. The subsequent if has_caption: then raises NameError. Additionally, the caption node reference (child) after the loop relied on silent loop-variable leakage — fragile and hard to read.

Fix: initialise has_caption = False and caption_child = None before the inner loop; capture the caption node in caption_child and use that variable after the loop.

---

2. License URL links broken in non-English locales (__init__.py:608, 812, 937)

figure_node["license"] stored the translated display value (e.g. German), but LICENSE_URLS is keyed by English names only. The in LICENSE_URLS look-up therefore always failed for non-English builds, so no license hyperlink was ever generated.

Fix: save the English (pre-translation) key in license_key_en immediately after untranslate_license() and store it as figure_node["license_key"]. _build_attribution_display now uses figure_node.get("license_key", figure_node["license"]) for the URL look-up.

---

3. ValueError crash in _copyright_from_authoryear (__init__.py:892, 897)

Both datetime.strptime(date_value, "%Y-%m-%d") calls had no exception handling. A date that arrives from a BibTeX year = field (e.g. "2023" or a partial date) and bypasses the directive-level validation crashes the build with an unhandled ValueError.

Fix: wrap both strptime calls in try/except ValueError; when the date cannot be parsed the year is simply omitted from the copyright string rather than raising.

Test plan

• Build with a non-English Sphinx locale (e.g. language = "de") and confirm license hyperlinks appear in the output HTML.
• Add a figure with no children (empty figure node) and confirm the build does not raise NameError.
• Add a BibTeX entry with year = {2023} (no month/day) and use it via :bib:; confirm the build completes and copyright is shown without a year.

https://claude.ai/code/session_01ApsmWPWsKBesMV1rq2VenA