fix: avoid marimo UI value access in math_eq_editor

[Mjboothaus]

Summary

• fix math_eq_editor(...) to avoid reading save_button.value in the cell that creates the UI element
• keep inline edit + save + preview behaviour while complying with marimo UI lifecycle rules
• add regression test coverage that fails if the helper tries to access button value in creator cell

Validation

• uv run ruff format src/pdealchemy/notebook_utils.py tests/notebook/test_notebook_utils.py
• uv run ruff check src/pdealchemy/notebook_utils.py tests/notebook/test_notebook_utils.py
• uv run ty check src
• uv run pytest tests/notebook/test_notebook_utils.py tests/notebook/test_notebook_spec.py tests/cli/test_notebook_to_toml.py
• uv run --extra interactive marimo check examples/notebooks/spec_black_scholes_with_results.py

Context

• Conversation: https://app.warp.dev/conversation/79aa7721-1947-44e5-840f-d20883bd8624

Co-Authored-By: Oz oz-agent@warp.dev

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
src/pdealchemy/notebook_utils.py	0% smaller
tests/notebook/test_notebook_utils.py	0% smaller