docs: add JSDoc for getError helper in createNetworkClient

Add comprehensive documentation for the getError function that extracts
errors from Cockatiel's FailureReason type in circuit breaker event handlers.
Documents both possible shapes of the FailureReason object.

Author: cryptodev-2s

packages/network-controller/src/create-network-client.ts

@@ -213,6 +213,23 @@ function createRpcServiceChain({
     logger,
   }));
 
+  /**
+   * Extracts the error from Cockatiel's `FailureReason` type received in
+   * circuit breaker event handlers.
+   *
+   * The `FailureReason` object can have two possible shapes:
+   * - `{ error: Error }` - When the RPC service throws an error (the common
+   * case for RPC failures).
+   * - `{ value: T }` - When the RPC service returns a value that the retry
+   * filter policy considers a failure.
+   *
+   * @param value - The event data object from the circuit breaker event
+   * listener (after destructuring known properties like `endpointUrl` and
+   * `primaryEndpointUrl`). This represents Cockatiel's `FailureReason` type.
+   * @returns The error or failure value, or `undefined` if neither property
+   * exists (which shouldn't happen in practice unless the circuit breaker is
+   * manually isolated).
+   */
   const getError = (value: object) => {
     if ('error' in value) {
       return value.error;