chore: adopt mypy 2.1.0 across Python libs

[John Kennedy (jkennedyvz)]

Summary

Adopts mypy 1.20.2 → 2.1.0 across all Python libs and fixes the type errors surfaced by the stricter checker. mypy 2.x landed via Dependabot's grouped major bumps but broke make lint in several libs; this PR adopts it cleanly in one place.

Only mypy and its transitive deps (librt, ast-serialize) change in the lockfiles — no runtime dependencies (redis, websockets, starlette, etc.) are touched. Every source change is type-only (annotations, cast, # type: ignore adjustments) with no runtime behaviour change.

Changes by lib

Lib	Fix
checkpoint-postgres	kwargs: dict[str, Any] before Item(**kwargs)
checkpoint-sqlite	params: list[Any] annotation
langgraph	cast(...) for the retry deprecation path in state.py/func (fixes 3 downstream object errors); cast into chained_fut in _runner.py; targeted [arg-type] ignores in binop.py/func for generic/ParamSpec limits; removed now-unused ignores in delta.py/message.py
prebuilt	cast("list[Any]", ...); narrowed an over-broad type: ignore
checkpoint, checkpoint-sqlite	mypy override for the optional, untyped langgraph.checkpoint.conformance.* test module + return-type annotations on the conformance tests
cli, sdk-py	mypy bump (sdk-py pin mypy==2.1.0); no source changes needed

Verification

make lint passes in all eight affected libs. Changes are type-only, so the test matrix is exercised by CI.

Relationship to open Dependabot PRs

• Supersedes chore(deps-dev): bump mypy from 1.20.2 to 2.1.0 in /libs/checkpoint-sqlite in the major group #7969, chore(deps-dev): bump mypy from 1.20.2 to 2.1.0 in /libs/checkpoint-postgres in the major group #7970, chore(deps-dev): bump mypy from 1.20.2 to 2.1.0 in /libs/prebuilt in the major group #7974 (pure mypy bumps) — close once this merges.
• Removes the mypy portion of the grouped bumps chore(deps): bump redis and starlette in /libs/langgraph #7976 (langgraph) and chore(deps): bump the major group in /libs/sdk-py with 2 updates #7972 (sdk-py); their remaining redis 7→8 / websockets 15→16 / starlette 0.51→1.2 majors should be assessed separately.

[Sydney Runkle (sydney-runkle)]

closing since we're migrating to ty