fix: convert astrolabe int provisioned_at to ISO before ProvisioningStatus

Round-4 review (real bug): get_background_sync_status now returns provisioned_at
as Unix seconds (the wire/pact value), but ProvisioningStatus.provisioned_at is
str | None (ISO). Constructing it for a provisioned user raised a Pydantic
ValidationError — a path that was unreachable before the has_access fix.

Convert int -> ISO at the oauth_tools boundary (mirroring the existing
refresh_token branch), keeping the model schema and the int-asserting contract
pact/unit tests intact. Add a regression test that drives the full
_get_provisioning_status round-trip with an integer timestamp.

Also surface dropped provider-state params in the verifier's _dispatch_state
no-op branch (round-4 nit).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/contract/test_mcp_provider_verification.py

@@ -86,7 +86,13 @@ def _dispatch_state(state: str, **kwargs) -> None:
     """
     handler = _PROVIDER_STATES.get(state)
     if handler is None:
-        logger.warning("No provider-state handler registered for %r; no-op", state)
+        # Log any params astrolabe passed so they're visible once real handlers
+        # need them (e.g. given("user X exists", params={"user_id": ...})).
+        logger.warning(
+            "No provider-state handler registered for %r (params=%s); no-op",
+            state,
+            kwargs,
+        )
         return
     handler()