docs: import AuthService in mocking tutorial

[fibibot]

Summary

• Move the real-world mocking tutorial AuthService example into an application module snippet.
• Update the test snippet to import AuthService from that module.
• Keep the test steps focused on credential validation, fetch stubbing, method spying, and fake time.

Verification

• deno fmt --check examples/tutorials/mocking.md
• Extracted auth_service.ts and auth_service_test.ts snippets from examples/tutorials/mocking.md, then ran deno test on the extracted test file
• deno task build:light, blocked by missing generated reference data: reference_gen/gen/deno.json

Fixes #2891
Closes bartlomieju/orchid-inbox#112

[lunadogbot]

The split into auth_service.ts and auth_service_test.ts fixes the missing import context, and the expiration step now awaits login() before advancing FakeTime. I checked FakeTime(new Date(...)) and the std testing imports against current Deno/std resolution. No issues found. Approving.

[lunadogbot]

@bartlomieju this is ready to merge