Run the tests on Node 22.16, and fix what a fresh setup tripped over

Node 22.16 runs TypeScript only behind --experimental-strip-types (on by
default from 22.18), so `npm test` failed on every file with
ERR_UNKNOWN_FILE_EXTENSION. The flag is harmless on newer versions.

probe and session-diagnose still imported dist/schulcloud/client.js, which
the move to core/ renamed, so both scripts died at import.

A relative MIRROR_DIR broke file serving: res.sendFile refuses a relative
path, and resolveWithin returns an absolute path only when its root is one.
The config resolves it once, at load.

package-lock.json is what `npm install` records today: the `schulcloud`
bin, and no stale peer flags. 101 tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -17,7 +17,7 @@
     "start": "node dist/bin/http.js",
     "stdio": "node dist/bin/stdio.js",
     "typecheck": "tsc -p tsconfig.json --noEmit",
-    "test": "node --test test/*.test.ts",
+    "test": "node --test --experimental-strip-types test/*.test.ts",
     "probe": "node --env-file=.env scripts/probe.mjs",
     "smoke": "node --env-file=.env scripts/smoke.mjs",
     "session-diagnose": "node --env-file=.env scripts/session-diagnose.mjs",