Fix thread safety of lazy graph object properties

[hb1915]

Link to issue

Closes #3441

Description of change

Make lazy initialization of compound graph-object properties safe for concurrent first reads. Concurrent readers now converge on the first child or child list published to the cache, preventing a later initializer from orphaning objects already returned to another thread.

Demo

Not applicable; this fixes an intermittent server-side exception without changing rendered figures.

Testing strategy

Added a deterministic two-thread regression test for both compound and compound-array properties. The test forces the failing interleaving: it fails on unpatched main with distinct compound children and ValueError("Invalid value") for the compound array, then passes with this change.

Validation performed:

• python -m pytest tests/test_core -q — 456 passed, 1 skipped
• Ruff 0.11.12 check and format check on the changed Python files
• Five end-to-end cold-template reproduction runs — 0 of 11 templates raced in every run

Additional information (optional)

The cache still permits duplicate construction during a concurrent cold read, but dict.setdefault makes publication single-winner so all readers receive objects that remain attached to their parent. This avoids adding a lock to every graph object and its associated memory, re-entrancy, deepcopy, and pickling considerations.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).