Refactor config to use test_default

Introduced a `test_default` method to `Config` to provide sane defaults
for integration tests. This refactors several test files to use this new
method, reducing boilerplate and improving maintainability.

Added `LLM_TIMEOUT_SECONDS` to the environment variables and `Config`
struct.

server/.env.example

@@ -28,6 +28,7 @@ LLM_URL=https://openai.ionos.com/openai
 LLM_API_KEY=
 LLM_MODEL=
 LLM_TEMPERATURE=0
+LLM_TIMEOUT_SECONDS=180
 
 # Session
 SESSION_TIMEOUT_HOURS=8