simplify: use direct envVars assignment in connectSandbox body

Matches the create-time pattern (envVars: opts?.envs) — openapi-fetch
strips undefined automatically so the conditional spread was unnecessary.

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

Author: FisherXZ

packages/js-sdk/src/sandbox/sandboxApi.ts

@@ -850,7 +850,7 @@ export class SandboxApi {
       },
       body: {
         timeout: timeoutToSeconds(timeoutMs),
-        ...(opts?.envs !== undefined ? { envVars: opts.envs } : {}),
+        envVars: opts?.envs,
       },
       signal: config.getSignal(opts?.requestTimeoutMs),
     })