Document non-empty api_key requirement

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

README.md

@@ -25,6 +25,8 @@ export HYPERBROWSER_BASE_URL="https://api.hyperbrowser.ai" # optional
 export HYPERBROWSER_HEADERS='{"X-Correlation-Id":"req-123"}' # optional JSON object
 ```
 
+`api_key` must be a non-empty string.
+
 `base_url` must start with `https://` (or `http://` for local testing), include a host,
 and not contain query parameters, URL fragments, or newline characters.
 The SDK normalizes trailing slashes automatically.