Evaluate migrating type checking from `mypy` to `ty`

[joshmarkovic]

Summary

Track migrating the type checker from mypy to ty (Astral's Rust-based checker), which would complete the move to a single Astral toolchain: Ruff for lint+format (#707 + the ruff format follow-up in #711), ty for types.

Motivation

1. Dependency friction is already biting. Commit 922b089 added override-dependencies = ["pathspec>=0.9,<1.1"] to resolve a mypy/pathspec conflict, and its own comment reads "Remove on dbt-core v1.12 or migration to ty by astral" — i.e. ty is already the flagged long-term fix.
2. Speed. On dbt/adapters: mypy 1.11.2 → 9.1s; ty 0.0.49 → 0.32s (~28×).
3. Toolchain consolidation. Ruff + ty is one Rust-based Astral toolchain with shared pyproject.toml config and fewer pre-commit hook repos / version pins to maintain.