test(api): stop tests leaking module mocks and env into later files

CI failed on a different set of tests each run because bun runs every test
file in one process and the file order differs between machines:

- the secretsmanager route test replaced the whole ../aws module, so files
  loaded after it lost awsClients, lambda and defaultAccountId (API Gateway,
  SES)
- the Lightsail tests restored FLOCI_ENDPOINT by assigning undefined, which
  stores the string "undefined" (runtime diagnostics)
- the auth tests used full-strength argon2 for ~30 sign-ins and timed out on
  slower runners; the OIDC cleanup test cached a discovery document without
  userinfo_endpoint for the shared issuer

Checked with 20 runs under bun test --randomize.

packages/api/src/services/lightsailDeployment.test.ts

@@ -5,7 +5,9 @@ const originalEndpoint = process.env.FLOCI_ENDPOINT
 const originalFetch = globalThis.fetch
 
 afterEach(() => {
-    process.env.FLOCI_ENDPOINT = originalEndpoint
+    // Assigning undefined would store the string "undefined" for every later test file.
+    if (originalEndpoint === undefined) delete process.env.FLOCI_ENDPOINT
+    else process.env.FLOCI_ENDPOINT = originalEndpoint
     globalThis.fetch = originalFetch
     mock.restore()
 })