[Config] `config set provider __custom__` can save an unusable config without `baseUrl`

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Description

The config set command validates provider names, but it does not enforce that the custom provider also has a base URL configured. This makes it easy to save a config that will only fail later at runtime.

Location

• src/commands/config.ts:53
• src/commands/config.ts:201

Problematic code

if (key === 'provider') {
  if (rawValue !== CUSTOM_PROVIDER_KEY && !getProviderInfo(rawValue)) {
    throw new Error(`Unknown provider: ${rawValue}.`);
  }
  return rawValue as ConfigSetValueMap[K];
}

Suggested fix

When setting provider to __custom__, either:

• require baseUrl to already be present, or
• print a follow-up warning/error until a valid custom endpoint is configured.

Impact

A user can save a configuration that looks valid but cannot create requests.