refactor(rpc-client): use DEFAULT_BASE_URL constant

Replace hard-coded 'https://api.stackone.com' fallback URL with the
DEFAULT_BASE_URL constant from consts.ts for consistency.

Author: ryoppippi

src/rpc-client.ts

@@ -1,4 +1,4 @@
-import { USER_AGENT } from './consts';
+import { DEFAULT_BASE_URL, USER_AGENT } from './consts';
 import { STACKONE_HEADER_KEYS } from './headers';
 import {
 	type RpcActionRequest,
@@ -26,7 +26,7 @@ export class RpcClient {
 
 	constructor(config: RpcClientConfig) {
 		const validatedConfig = rpcClientConfigSchema.parse(config);
-		this.baseUrl = validatedConfig.serverURL || 'https://api.stackone.com';
+		this.baseUrl = validatedConfig.serverURL || DEFAULT_BASE_URL;
 		const username = validatedConfig.security.username;
 		const password = validatedConfig.security.password || '';
 		this.authHeader = `Basic ${Buffer.from(`${username}:${password}`).toString('base64')}`;