refactor(models): consolidate artifact and dependency handling

[tulayha]

Summary

• Centralize retry, backoff, error translation, checksum verification, and preparation progress used by model providers.
• Make model readiness require the pinned checksum instead of accepting any file at the expected path.
• Update the locked GitPython version from 3.1.57 to 3.1.58 for its security fixes.
• Keep provider-specific model constructors inside their capabilities.

This is internal-only. It does not change public contracts and requires no model, index, or repository rebuild.

Validation

• uv run --no-sync ruff check .
• uv run --no-sync pytest -q tests/test_models.py — 30 passed, 2 subtests passed.
• $env:PYTHONPATH='src'; uv run --no-sync python -m unittest discover -s tests -q — 612 passed, 4 skipped with GitPython 3.1.58 installed.
• uv lock --check