PR #1837's new `_kanban_unknown_endpoint` wrapper was triggered for any
falsy bridge return — but `handle_kanban_*` returns `None` (not `True`)
when an inner handler calls `bad(...)` to send an error response. The
wrapper then sent a SECOND 404 on top of the bridge's response, producing
concatenated JSON bodies on the wire.
Concrete reproducer (caught by behavioural harness, not the merged tests):
GET /api/kanban/tasks/<missing-id>/log
→ '{"error":"task not found"}{"error":"unknown Kanban endpoint: GET ..."}'
This affected every `bad(...)`-shaped error path in the bridge:
- task-not-found returns from `_task_log_payload` / `_task_detail_payload`
- exception handlers for ImportError (503), LookupError (404),
ValueError (400), RuntimeError (409) across all four method handlers
- the `_handle_events_sse_stream` board-resolution failure path
The fix: distinguish an explicit `False` (truly unmatched path) from
`None` (handled, response already sent). Only `False` should trigger
the unknown-endpoint diagnostic.
Adds a regression test that exercises the task-not-found path through
`routes.handle_get` and asserts only one JSON body is on the wire.
Follow-on to #1837 (already merged into master at v0.51.20).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
