feat(providers): add warmup() for OpenAI, Anthropic, Gemini, Compatible, GLM

All five providers have HTTP clients but did not implement warmup(),
relying on the trait default no-op. This adds lightweight warmup calls
to establish TLS + HTTP/2 connection pools on startup, reducing
first-request latency. Each warmup is skipped when credentials are
absent, matching the OpenRouter pattern.

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

src/providers/openai.rs

@@ -322,6 +322,18 @@ impl Provider for OpenAiProvider {
     fn supports_native_tools(&self) -> bool {
         true
     }
+
+    async fn warmup(&self) -> anyhow::Result<()> {
+        if let Some(credential) = self.credential.as_ref() {
+            self.client
+                .get("https://api.openai.com/v1/models")
+                .header("Authorization", format!("Bearer {credential}"))
+                .send()
+                .await?
+                .error_for_status()?;
+        }
+        Ok(())
+    }
 }
 
 #[cfg(test)]
@@ -437,4 +449,11 @@ mod tests {
         let resp: ChatResponse = serde_json::from_str(&json).unwrap();
         assert_eq!(resp.choices[0].message.content.len(), 100_000);
     }
+
+    #[tokio::test]
+    async fn warmup_without_key_is_noop() {
+        let provider = OpenAiProvider::new(None);
+        let result = provider.warmup().await;
+        assert!(result.is_ok());
+    }
 }