fix: resolve failing test, 50 ruff lint errors, and missing pytest-cov dependency

[badgers4018]

Summary

• Failing test fix (test_imports_from_cells): The sample_notebook.ipynb fixture was missing import statements (os, pathlib, math) that the test expected to find as IMPORTS_FROM edges. Added import os and from pathlib import Path to cell 1 (alongside existing magic lines) and import math to cell 2, preserving all existing cell index assertions.

• Ruff lint errors (50 total): Auto-fixed 12 errors (import sorting in test_tools.py, unused shutil import, trailing newline in fixture). Configured proper exclusions and per-file-ignores for the remaining 38 errors:

  • Excluded diagrams/ directory — DSL scripts use intentionally compact, non-standard style (semicolons, single-letter function names, long lines)
  • Excluded sample_databricks_notebook.ipynb — SQL/R/Scala cells produce syntax errors when parsed as Python
  • Per-file-ignores for sample_databricks_export.py (unused variable, trailing newline in fixture)
  • Per-file-ignores for sample_notebook.ipynb (intentionally unused imports, cross-cell import blocks)
  • Per-file-ignores for test_multilang.py (long assertion lines with explanatory comments)

• Missing dependency: Added pytest-cov to the [project.optional-dependencies] dev group, required for --cov coverage reporting.

Test plan

• All 620 tests pass (pytest — 620 passed, 2 xpassed)
• ruff check . reports zero errors
• test_imports_from_cells specifically passes with correct import targets
• test_cell_index_tracking still passes (cell indices unchanged)
• test_skips_magic_commands still passes (magic line filtering unaffected)

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