refactor: align with Bub contract boundaries

[PsiACE]

Summary

• import Bub contracts from their subsystem-owned modules introduced by refactor: align contracts with subsystem boundaries bub#260
• rename turn state, model selection, and channel router usage to the new APIs
• update ACP model option conversion and channel routing helpers
• update affected tests across the contrib workspace

Dependency

uv.lock tracks the merged bubbuild/bub#260 revision at merge commit 2dd63a0.

Testing

• uv lock --locked
• uv run --frozen --group test pytest on Python 3.12 (316 passed, 2 skipped)
• uv run --frozen ruff check .
• uv build --all-packages