Deduplicate TOML rendering and spec test fixtures

[Mjboothaus]

Summary

• extract shared TOML string rendering into src/pdealchemy/toml_rendering.py
• reuse the shared renderer in notebook_spec and spec_bridge
• add shared write_spec_toml pytest fixture in tests/conftest.py and remove duplicated test helper implementations

Validation

• uv run ruff check src/pdealchemy/notebook_spec.py src/pdealchemy/spec_bridge.py src/pdealchemy/toml_rendering.py tests/conftest.py tests/notebook/test_spec_bridge.py tests/cli/test_spec_to_runtime_toml.py
• uv run ty check src
• uv run pytest tests/notebook/test_notebook_spec.py tests/cli/test_notebook_to_toml.py tests/notebook/test_spec_bridge.py tests/cli/test_spec_to_runtime_toml.py

Conversation: https://app.warp.dev/conversation/79aa7721-1947-44e5-840f-d20883bd8624
Plan: https://app.warp.dev/drive/notebook/eZQFltrUPlV50R8ZwHEybR

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

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
tests/notebook/test_spec_bridge.py	24% smaller
tests/cli/test_spec_to_runtime_toml.py	16% smaller
src/pdealchemy/notebook_spec.py	0% smaller
src/pdealchemy/spec_bridge.py	0% smaller
src/pdealchemy/toml_rendering.py	0% smaller
tests/conftest.py	0% smaller