test: make dbt and warehouse tests hermetic

[C00ldudeNoonan]

Summary

• make dbt dependency installation explicit and parse the project once per test session
• block network access in the default pytest suite while keeping SEC functional tests opt-in
• close Dagster and DuckDB test resources deterministically
• add SQLFluff and critical-path coverage enforcement to CI

Validation

• 454 tests passed with 27 network tests deselected
• critical-path coverage: 51.69% against a 50% floor
• Ruff, ty, SQLFluff, actionlint, and Dagster definition validation passed
• opt-in SEC functional suite: 27 passed

Closes #134

[C00ldudeNoonan]

@codex please review this PR.

[chatgpt-codex-connector]

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