🔧 Move test lint disables to oxlint config

The no-floating-promises header was repeated at the top of every test
file. Centralize it, and the no-null suppression the new null/undefined
cases need, in the **/*.test.ts override.

src/util/__tests__/lsp-completion.test.ts

@@ -1,4 +1,3 @@
-/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync type-assertion library that the type-aware linter misidentifies as a promise */
 import { strict as assert } from "node:assert";
 import path from "node:path";
 import { test } from "node:test";