fix: stabilize secrets scanner runs

[hurcy]

Summary

• Avoid no-op Delta table and column comment writes during SAT initialization, preventing concurrent metadata conflicts.
• Run notebook and cluster secret scans sequentially within a workspace on serverless compute.
• Exclude compiled Python cache artifacts from source scanning and record genuine materialization coverage gaps in a dedicated Delta table.
• Include child scan details in the driver failure message.

Fixes #391.

Validation

• python3 -m py_compile passed for all modified notebooks.
• git diff --check passed.
• codespell passed on staged files.
• Validated against a Databricks serverless workspace: Secrets Scanner completed successfully with zero coverage gaps after excluding __pycache__ .pyc files.

Test note

pytest tests/ requires an external config file containing a MEISTERSTUFF section. In a clean checkout it collected 193 tests but 192 stopped at fixture setup because /path/to/config.txt was absent; this is unrelated to the modified notebook code.

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