refactor(adapters): implement polymorphic PipelineAdapter interface (#145)

[tjirab]

🚀 What changed?

• Introduced PipelineAdapter abstract base class in tff.core.adapter defining unified lifecycle methods: provider_name, is_applicable, load_models, run_checks, apply_metadata_fix, and get_diagnostic_files.
• Implemented concrete adapters:
  • DBTAdapter in tff.dbt.adapter
  • SQLMeshAdapter in tff.sqlmesh.adapter
  • DataformAdapter in tff.dataform.adapter
• Updated runner modules (dbt/runner.py, sqlmesh/runner.py, dataform/runner.py) to accept an optional preloaded models: dict[str, ModelRepresentation] | None = None.
• Refactored tff.core.docs.generate_docs_dashboard to eliminate procedural branching and eliminate duplicate manifest/context model loading by reusing preloaded models in run_checks.
• Refactored tff.core.autofix.apply_autofixes to dispatch metadata fixes polymorphically via adapter.apply_metadata_fix(...) instead of hardcoded provider checks.
• Refactored tff.core.cli to use detect_provider and _get_adapter(provider), eliminating provider ladders in info, lint, and health, while introducing _MockRunnerAdapter to preserve complete backwards compatibility for existing tests mocking _get_runner.
• Added comprehensive unit tests in packages/tff-core/tests/test_adapter.py achieving 100% diff test coverage.

🤔 Why is this change needed?

Resolves issue #145:

• In tff-core, CLI commands (info, lint, health), docs.py, and autofix.py contained hardcoded if provider == "dbt" ... elif provider == "sqlmesh" ... elif provider == "dataform" procedural ladders. Adding support for a new transformation engine required editing multiple disjoint files across the codebase.
• In tff docs and tff lint --fix, models were compiled and loaded twice (first explicitly for mapping, and a second time inside the runner check execution), causing unnecessary compilation latency.

🔗 Related Issues (Optional)

Closes #145

✅ Next Steps / Checklist (Optional)

• Abstract PipelineAdapter base class and provider registry implemented
• Concrete DBTAdapter, SQLMeshAdapter, and DataformAdapter implemented
• Runner checks accept pre-loaded models to eliminate duplicate compilation
• Procedural ladders removed from CLI, docs, and autofix
• Backwards compatibility maintained for existing tests and mocking patterns
• 100% diff coverage verified
• All 267 unit tests pass