fix: use correct Browserbase API to release sessions (#707)

Browserbase has no DELETE endpoint for sessions. The correct API is
POST /v1/sessions/:id with body { status: "REQUEST_RELEASE" }. The old
DELETE call returned an error that was silently swallowed, causing every
session to leak until the 30-min idle timeout.

Fixed in both Node.js (src/browser.ts) and native Rust
(cli/src/native/providers.rs) paths.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

cli/src/native/providers.rs

@@ -35,11 +35,13 @@ pub async fn close_provider_session(session: &ProviderSession) {
         "browserbase" => {
             if let Ok(api_key) = env::var("BROWSERBASE_API_KEY") {
                 let _ = client
-                    .delete(format!(
+                    .post(format!(
                         "https://api.browserbase.com/v1/sessions/{}",
                         session.session_id
                     ))
+                    .header("Content-Type", "application/json")
                     .header("X-BB-API-Key", &api_key)
+                    .json(&serde_json::json!({ "status": "REQUEST_RELEASE" }))
                     .send()
                     .await;
             }