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/gateway/test_choreographer_pm_extras.py

@@ -379,6 +379,7 @@ async def test_delegate_main_pm_to_cell_pm_creates_subtask() -> None:
             description="Plan backend work for feature X",
             assigned_to="be-pm",
             team="backend",
+            task_type="planning",
         ),
     )
     assert env.error is None
@@ -417,6 +418,7 @@ async def test_delegate_cell_pm_to_team_dev_creates_subtask() -> None:
             description="Add /v1/foo endpoint with tests",
             assigned_to="be-dev-1",
             team="backend",
+            task_type="code",
         ),
     )
     assert env.error is None
@@ -438,7 +440,11 @@ async def test_delegate_main_pm_to_dev_is_rejected() -> None:
         main_pm_id,
         parent_id,
         DelegateInputs(
-            title="x", description="y", assigned_to="be-dev-1", team="backend"
+            title="x",
+            description="y",
+            assigned_to="be-dev-1",
+            team="backend",
+            task_type="code",
         ),
     )
     body = env.as_dict()
@@ -460,7 +466,13 @@ async def test_delegate_cell_pm_to_other_pm_rejected() -> None:
     env = await c.delegate(
         cell_pm_id,
         parent_id,
-        DelegateInputs(title="x", description="y", assigned_to="be-pm", team="backend"),
+        DelegateInputs(
+            title="x",
+            description="y",
+            assigned_to="be-pm",
+            team="backend",
+            task_type="code",
+        ),
     )
     body = env.as_dict()
     assert body["error"] == "not_authorized"
@@ -481,7 +493,11 @@ async def test_delegate_unknown_assignee_returns_invalid_state() -> None:
         pm_id,
         parent_id,
         DelegateInputs(
-            title="x", description="y", assigned_to="nope-pm", team="backend"
+            title="x",
+            description="y",
+            assigned_to="nope-pm",
+            team="backend",
+            task_type="code",
         ),
     )
     body = env.as_dict()
@@ -503,7 +519,11 @@ async def test_delegate_invalid_team_enum_rejected() -> None:
         pm_id,
         parent_id,
         DelegateInputs(
-            title="x", description="y", assigned_to="be-dev-1", team="not-a-team"
+            title="x",
+            description="y",
+            assigned_to="be-dev-1",
+            team="not-a-team",
+            task_type="code",
         ),
     )
     assert env.as_dict()["error"] == "invalid_state"