Add optional API user support for token authentication

- Add user field to apiTokenAuth struct
- Add WithAPITokenUser(token, user) method for custom username
- Default to "jsonrpc" when no user specified (backward compatible)
- Add tests for custom user and empty user scenarios

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client.go

@@ -38,11 +38,19 @@ func NewClient(baseURL string) *Client {
 }
 
 // WithAPIToken configures the client to use API token authentication.
+// Uses "jsonrpc" as the username for HTTP Basic Auth.
 func (c *Client) WithAPIToken(token string) *Client {
 	c.auth = &apiTokenAuth{token: token}
 	return c
 }
 
+// WithAPITokenUser configures the client to use API token authentication with a custom username.
+// If user is empty, "jsonrpc" will be used as the default.
+func (c *Client) WithAPITokenUser(token, user string) *Client {
+	c.auth = &apiTokenAuth{token: token, user: user}
+	return c
+}
+
 // WithBasicAuth configures the client to use username/password authentication.
 func (c *Client) WithBasicAuth(username, password string) *Client {
 	c.auth = &basicAuth{username: username, password: password}