fix(a2a): export Checkpoint, switch _process_task_response to TaskState enum

- Export ``Checkpoint`` TypedDict from the top-level ``adcp`` package
  so callers can ``from adcp import Checkpoint`` instead of reaching
  into ``adcp.client`` for the persistence-key type.
- Compare ``task.status.state`` against ``TaskState.completed`` /
  ``TaskState.failed`` in ``_process_task_response`` to match the
  enum-based membership check in ``_NONTERMINAL_TASK_STATES`` — an
  upstream rename in a2a-sdk now fails as a type error instead of a
  silent classification drift.

Also flips the in-progress release-please PR from 5.0.0 to 4.0.1. The
prior commit (#258) landed breaking-style changes (``pending_task_id``
→ ``active_task_id``, ``ValueError`` → ``TypeError`` on non-A2A) but
4.0.0 shipped under 48h earlier with no known A2A adopters — the
salesagent migration is on MCP, and A2A adoption is deferred behind
pluggable TaskStore / push-notification work. Treating those as
pre-adoption corrections rather than a major bump.

Release-As: 4.0.1

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

src/adcp/__init__.py

@@ -27,7 +27,7 @@
     build_synthetic_capabilities,
     validate_capabilities,
 )
-from adcp.client import ADCPClient, ADCPMultiAgentClient
+from adcp.client import ADCPClient, ADCPMultiAgentClient, Checkpoint
 from adcp.exceptions import (  # noqa: F401
     AdagentsNotFoundError,
     AdagentsTimeoutError,
@@ -566,6 +566,7 @@ def get_adcp_version() -> str:
     # Client classes
     "ADCPClient",
     "ADCPMultiAgentClient",
+    "Checkpoint",
     "RegistryClient",
     "PropertyRegistry",
     "RegistrySync",

src/adcp/protocols/a2a.py

@@ -523,7 +523,7 @@ def _process_task_response(self, task: Task, debug_info: DebugInfo | None) -> Ta
         """Process a Task response from A2A into our TaskResult format."""
         task_state = task.status.state
 
-        if task_state == "completed":
+        if task_state == TaskState.completed:
             # Extract the result from the artifacts array
             result_data = self._extract_result_from_task(task)
 
@@ -542,7 +542,7 @@ def _process_task_response(self, task: Task, debug_info: DebugInfo | None) -> Ta
                 },
                 debug_info=debug_info,
             )
-        elif task_state == "failed":
+        elif task_state == TaskState.failed:
             # Protocol-level failure - extract error message from TextPart
             error_msg = self._extract_text_from_task(task) or "Task failed"
             return TaskResult[Any](