fix: Use orElseGet to defer loading of default credentials

[mrdziuban]

When constructing a Client with explicitly provided credentials, the ApiClient constructor still throws an error about failing to get application default credentials. For example:

Client client = Client.builder()
  .credentials(...)
  .project("...")
  .location("...")
  .vertexAI(true)
  .build();
/*
java.lang.IllegalArgumentException: Failed to get application default credentials, please explicitly provide credentials.
	at com.google.genai.ApiClient.<init>(ApiClient.java:109)
	at com.google.genai.HttpApiClient.<init>(HttpApiClient.java:43)
	at com.google.genai.Client.<init>(Client.java:203)
	at com.google.genai.Client.<init>(Client.java:27)
	at com.google.genai.Client$Builder.build(Client.java:55)
	...
*/

As a fix, this defers loading of the application default credentials by calling orElseGet instead of orElse on the given credentials.

[mrdziuban]

@jaycee-li could you take a look at this? Or if not, could you tag someone who can? Thank you!

[jaycee-li]

Thanks for the fix!