Parallel subtests in `coderd/x/chatd` reused a parent test context with
a `testutil.WaitLong` deadline, so the context could expire before a
subtest was scheduled under load. That made the subagent lifecycle tools
return plain-text context errors instead of the expected JSON payload,
causing flaky JSON unmarshal failures.
Create fresh `chatdTestContext` values inside the affected parallel
subtests and add `chatdTestContext` to the `paralleltestctx` custom
function list so this pattern is caught by `make lint`.
Closes https://github.com/coder/internal/issues/1494
Ethan
063c06ca5f