fix(gateway): unblock PM planning + drop magic delegate task_type

Two coupled fixes from the 2026-05-08 smoke-test trace:

1. pm_cannot_execute_code is now scoped to i_will_work_on (the
   EXECUTION verb) only. Pre-fix it also fired on i_will_plan, which
   deadlocked any code-typed parent: cell_pm couldn't plan, so couldn't
   transition parent to in_progress, so couldn't delegate. PMs PLAN
   code-typed parents and DELEGATE the work — that's exactly the verb
   we were blocking.

2. delegate.task_type is now REQUIRED at both the HTTP boundary
   (DelegateRequest) and the choreographer dataclass (DelegateInputs).
   The pre-fix default of 'code' silently changed semantics whenever a
   caller forgot the field — main-pm's call in the smoke trace omitted
   it, schema defaulted to 'code', and the cell PM downstream was
   wedged. Also drops the choreographer's task.task_type fallback
   (the DB column is NOT NULL anyway).

Plus middleware coverage tests for the parallel ServiceError →
4xx handler hierarchy added in the prior session, restoring 100%
coverage across the touched files.

Tests: 3101 passing, 100% coverage, ruff clean.

tests/unit/api/test_schemas_v2_flow.py

@@ -0,0 +1,42 @@
+"""Schema-level tests for v2 flow request bodies."""
+
+from __future__ import annotations
+
+from uuid import uuid4
+
+import pytest
+from pydantic import ValidationError
+from roboco.api.schemas.v2.flow import DelegateRequest
+
+
+def test_delegate_request_requires_task_type() -> None:
+    """task_type must be supplied explicitly — no magic default.
+
+    Background: the 2026-05-08 smoke-test trace showed main-pm calling
+    delegate without task_type, the schema defaulted to 'code', the
+    cell PM downstream couldn't plan a code-typed parent (pre-fix), and
+    the run deadlocked. Make the field required so misuse fails at the
+    HTTP boundary with a clear 422.
+    """
+    with pytest.raises(ValidationError) as exc:
+        DelegateRequest(
+            parent_task_id=uuid4(),
+            title="t",
+            description="d",
+            assigned_to="be-dev-1",
+            team="backend",
+            # task_type intentionally omitted
+        )
+    assert "task_type" in str(exc.value)
+
+
+def test_delegate_request_accepts_explicit_task_type() -> None:
+    req = DelegateRequest(
+        parent_task_id=uuid4(),
+        title="t",
+        description="d",
+        assigned_to="be-dev-1",
+        team="backend",
+        task_type="code",
+    )
+    assert req.task_type == "code"