feat: add configurable provider timeout

[Fybre]

Add requestTimeoutS field to agent defaults config (default 60s).

Allows users to increase timeout for slow models or congested providers.

Example config:

{
  "agents": {
    "defaults": {
      "requestTimeoutS": 120
    }
  }
}

[louisho5]

Thanks @Fybre, this is a good idea for slow models on cheap hardware :)

However, the PR changed NewOpenAIProvider to take 3 arguments instead of 2, so some functions also need to be updated.

What I have done:

Updated NewOpenAIProvider to take 3 arguments.
# main.go (agent command)
# openai_test.go (test)
# llm_ranker_provider_integration_test.go (integration test)