fix: propagate warmup errors and skip when no API key configured

Address review feedback from @coderabbitai and @gemini-code-assist:
- Missing API key is now a silent no-op instead of returning an error
- Network/TLS errors are now propagated via `?` instead of silently
  discarded, so they surface as non-fatal warnings in the caller's log
- Added `error_for_status()` to catch HTTP-level failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/providers/openrouter.rs

@@ -54,16 +54,14 @@ impl Provider for OpenRouterProvider {
     async fn warmup(&self) -> anyhow::Result<()> {
         // Hit a lightweight endpoint to establish TLS + HTTP/2 connection pool.
         // This prevents the first real chat request from timing out on cold start.
-        let api_key = self
-            .api_key
-            .as_ref()
-            .ok_or_else(|| anyhow::anyhow!("No API key for warmup"))?;
-        let _ = self
-            .client
-            .get("https://openrouter.ai/api/v1/auth/key")
-            .header("Authorization", format!("Bearer {api_key}"))
-            .send()
-            .await;
+        if let Some(api_key) = self.api_key.as_ref() {
+            self.client
+                .get("https://openrouter.ai/api/v1/auth/key")
+                .header("Authorization", format!("Bearer {api_key}"))
+                .send()
+                .await?
+                .error_for_status()?;
+        }
         Ok(())
     }