Test hardening for auth token + multimodal inputs

[chindris-mihai-alexandru]

Tasks:

• Add MSW handler for /auth/get-user-app-token to silence warnings.
• Add integration tests covering image_url and PDF (puter_path) inputs.
• Add caching test coverage for cache hit/miss behavior.

Acceptance:

• Tests cover auth token endpoint, multimodal inputs, and caching paths.