chore: clippy fixes for 0.1.2

- #[allow(clippy::too_many_arguments)] on OpenRouterClient::call_once
  (8 args after audit-passthrough plumbing; refactor would distort the
  request struct shape for a single-call-site lint nudge — not worth it)
- `parsed.choices.iter().next()` → `.first()` per clippy::iter_next_slice

Both surfaced by CI's `cargo clippy --workspace --all-targets -- -D warnings`
on the v0.1.2 bump commit; local `cargo build` doesn't gate on warnings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: enriquephl

crates/eros-engine-llm/src/openrouter.rs

@@ -201,6 +201,7 @@ impl OpenRouterClient {
         }
     }
 
+    #[allow(clippy::too_many_arguments)]
     async fn call_once(
         &self,
         model: &str,
@@ -242,8 +243,7 @@ impl OpenRouterClient {
         let parsed: WireResponse = resp.json().await?;
         let raw = parsed
             .choices
-            .iter()
-            .next()
+            .first()
             .and_then(|c| c.message.content.clone())
             .unwrap_or_default();
         Ok(ChatResponse {