feat: validate graph connections during drag

[philippjfr]

Description

I added opt-in browser checks for direction, type, input capacity, duplicates, and cycles, plus Python validators that evaluate candidate handles when a drag starts and show rejection reasons in the canvas. This lets applications prevent invalid interactive connections before creating an edge; Python validators do not restrict programmatic edges, so state-dependent rules still need checking in edge_added. I added a runnable demo, a how-to guide, and API/Playwright coverage. No new dependencies or migration steps are required.

To reproduce, run PYTHONPATH=src pixi run panel serve examples/connection_validation.py --show, then drag Source.text to Transform.text (allowed), Number.value to Transform.text (type mismatch), and Source.text to Publish.text (Python rejection).

Before: invalid drags could create edges and required application-side rollback. After: rejected candidates show their reason during dragging and the browser blocks the drop. The demo above shows both states of the new handle feedback.

Verification: pixi run compile, PYTHON_GIL=1 PYTHONPATH=src pixi run pytest -q (169 passed, 4 skipped), pixi run -e lint ruff check src/panel_reactflow/base.py tests/test_api.py tests/ui/test_ui.py, pixi run -e docs docs-build, and commit hooks passed. Two focused Playwright tests reported 2 passed, 35 deselected; the process then timed out because Bokeh session-cleanup callbacks continued after pytest reported its result. The full browser suite did not finish here.

AI Disclosure

Tool & Model: Kilo Code + openai/gpt-6-sol.
Usage: AI drafted the connection-validation how-to guide, linked it in the docs, reviewed the existing feature changes, ran verification, and prepared this PR.

• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing (Python suite and focused browser tests; full browser suite not completed due to cleanup timeout)
• Added documentation