refactor: `llm_call_info_var` relies on callers pre-creating `LLMCallInfo`

`llm_call()` assumes `llm_call_info_var` is already set with a `task` field by the caller. Nothing enforces this. Per #1616 review comments we have following suggestion:

Pass a mandatory `Task` enum to `llm_call()` and have `@track_llm_call` create the `LLMCallInfo` internally instead of mutating a pre-existing one.
