feat(githubcopilot): add ability to fetch supported models (#2717)

Signed-off-by: Gustavo Hoirisch <gugahoi@users.noreply.github.com>

crates/goose/src/providers/githubcopilot.rs

@@ -424,4 +424,45 @@ impl Provider for GithubCopilotProvider {
         emit_debug_trace(&self.model, &payload, &response, &usage);
         Ok((message, ProviderUsage::new(model, usage)))
     }
+
+    /// Fetch supported models from GitHub Copliot; returns Err on failure, Ok(None) if not present
+    async fn fetch_supported_models_async(&self) -> Result<Option<Vec<String>>, ProviderError> {
+        let (endpoint, token) = self.get_api_info().await?;
+        let url = format!("{}/models", endpoint);
+
+        let mut headers = http::HeaderMap::new();
+        headers.insert(http::header::ACCEPT, "application/json".parse().unwrap());
+        headers.insert(
+            http::header::CONTENT_TYPE,
+            "application/json".parse().unwrap(),
+        );
+        headers.insert("Copilot-Integration-Id", "vscode-chat".parse().unwrap());
+        headers.insert(
+            http::header::AUTHORIZATION,
+            format!("Bearer {}", token).parse().unwrap(),
+        );
+
+        let response = self.client.get(url).headers(headers).send().await?;
+
+        let json: serde_json::Value = response.json().await?;
+
+        let arr = match json.get("data").and_then(|v| v.as_array()) {
+            Some(arr) => arr,
+            None => return Ok(None),
+        };
+        let mut models: Vec<String> = arr
+            .iter()
+            .filter_map(|m| {
+                if let Some(s) = m.as_str() {
+                    Some(s.to_string())
+                } else if let Some(obj) = m.as_object() {
+                    obj.get("id").and_then(|v| v.as_str()).map(str::to_string)
+                } else {
+                    None
+                }
+            })
+            .collect();
+        models.sort();
+        Ok(Some(models))
+    }
 }