feat(python): support callable file providers

[chaliy]

Summary

• accept str | Callable[[], str] in Python files= for both Bash and BashTool
• materialize callable-backed files lazily on first read and cache the result in the VFS
• surface provider exceptions through command execution and preserve TypeError for non-string direct reads

Closes #1258

Verification

• ./.venv/bin/python -m pytest tests/test_bashkit.py -q
• uvx ruff check crates/bashkit-python
• uvx ruff format --check crates/bashkit-python
• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• smoke: lazy providers via Bash.read_file() and BashTool.execute_sync("cat ...")