Fix issue #215: stabilize heavy ipyleaflet rerenders

[cpsievert]

Summary

• fixes heavy ipyleaflet rerenders by opening child widget models before opening the parent widget model
• upgrades the #215 Playwright regression to use a LayerGroup with 1,000 CircleMarkers so it matches the reported heavy-map failure mode
• keeps the browser-side widget manager logic unchanged so the diff stays focused on the ordering bug itself

Problem

Issue #215 reports that rerendering an ipyleaflet map with many points can leave the map gray or unusable.

The failure mode here is that a parent widget view can start rendering before the browser knows about all child widget models referenced by that parent state. In ipyleaflet, that shows up as child-view creation failures and incomplete map rendering when a map has many layers.

Fix

This change keeps the existing delayed-open behavior for widget initialization, but narrows the ordering fix to one specific rule on the Python side:

• before opening a widget comm, inspect that widget's serialized state
• recursively open any referenced child widget models first
• only then open the parent widget model

That preserves child-before-parent availability during initial render and rerender while keeping the browser-side code unchanged.

Evidence

I re-ran this from a clean origin/main worktree to make sure the final version still has a real red/green story.

Baseline with only the new heavy #215 repro applied:

• tests/playwright/ipyleaflet_rerender_cleanup -vv fails on origin/main
• the map container renders, but no path.leaflet-interactive elements ever appear within the 30s timeout

Then, applying only the _shinywidgets.py change from this branch on top of that clean baseline:

• tests/playwright/ipyleaflet_rerender_cleanup -vv passes
• tests/playwright/ipyleaflet_marker_click -vv passes
• tests/playwright/plotly_rerender_cleanup -vv passes
• tests/playwright/altair_rerender_cleanup -vv passes
• tests/playwright/bokeh_rerender_cleanup -vv passes

That makes the current confidence story much stronger than the earlier exploratory versions of this branch: the heavy repro is red on baseline, and it turns green with this narrower Python-side fix alone.

Verification

Verified locally on this branch with:

• yarn build
• ./.venv/bin/python -m pytest -c tests/playwright/playwright-pytest.ini tests/playwright/ipyleaflet_marker_click -vv
• ./.venv/bin/python -m pytest -c tests/playwright/playwright-pytest.ini tests/playwright/ipyleaflet_rerender_cleanup -vv
• ./.venv/bin/python -m pytest -c tests/playwright/playwright-pytest.ini tests/playwright/plotly_rerender_cleanup -vv
• ./.venv/bin/python -m pytest -c tests/playwright/playwright-pytest.ini tests/playwright/altair_rerender_cleanup -vv
• ./.venv/bin/python -m pytest -c tests/playwright/playwright-pytest.ini tests/playwright/bokeh_rerender_cleanup -vv