fix(a2a): Checkpoint export + TaskState enum cleanup, pin release as 4.0.1

[bokelley]

Summary

Two small follow-ups to PR #258, plus a `Release-As: 4.0.1` footer to override the pending 5.0.0 release PR (#257).

Code changes

• Export `Checkpoint` from top-level `adcp` — callers can now `from adcp import Checkpoint` instead of `from adcp.client import Checkpoint`, which matters because it's the type used as a persistence-key dict.
• Enum comparisons in `_process_task_response` — `task_state == TaskState.completed` / `TaskState.failed` matches the enum-based frozenset used for non-terminal states. An upstream rename in a2a-sdk now becomes a type error instead of a silent classification drift.

Release positioning: 4.0.1 not 5.0.0

PR #258 landed with `feat(a2a)!:` + `BREAKING CHANGE:` footer, so release-please opened #257 for 5.0.0. The breaking bits were:

• `pending_task_id` → `active_task_id` (public property rename)
• `ValueError` → `TypeError` on non-A2A `context_id=` / `reset_context()`

4.0.0 shipped <48h before #258 landed. Per the salesagent roadmap, MCP is the critical path; A2A adoption is deferred behind pluggable TaskStore / push-notification work that isn't merged yet. Treating these as pre-adoption corrections rather than a major bump.

What happens after merge

Once this lands on main, release-please will regenerate #257 at 4.0.1 instead of 5.0.0. Merge that and PyPI publishes 4.0.1.

Test plan

• `ruff check src/` — clean
• `mypy src/adcp/` — 684 files, clean
• `pytest tests/test_protocols.py tests/integration/test_a2a_context_id.py` — 70/70 pass

Heads-up, not blocking 4.0.1

• a2a-sdk 1.0.1 is out (our pin is `<1.0`). Not backward-compatible — types moved, `DefaultRequestHandler` renamed, `ServerError` removed, `Part`/`Message` construction changed. Repo already tracks this as a separate compat PR (see pyproject.toml comment at a2a-sdk line). The `A2AClient is deprecated` warnings in our test output are the 0.3.x line signalling the 1.0 migration.

🤖 Generated with Claude Code

[bokelley]

Folding into the consolidated 4.1.0 a2a-sdk 1.0 migration PR. Both changes (Checkpoint top-level export, TaskState enum comparisons in _process_task_response) will land there.