fix: authentication failures when using Google Cloud universes

[jnahelou]

Description

This PR fixes an authentication issue when using custom universe domains with a tokenSource.

When initializing a tokenSource, the universe domain must be explicitly passed to it. Otherwise, it falls back to the default Google universe domain (googleapis.com), causing a credential mismatch error and failing to authenticate requests.

In the same way, when using DefaultTokenSource, the universe domain was not properly propagated, causing the same issue.
To resolve this, I switched to using FindDefaultCredentials, which natively includes proper support for custom universe domains and correctly parses the environment's credentials.

Related Logs / Error Output

When the universe domain is not explicitly configured on the tokenSource, the following error occurs during the project metadata retrieval:

2026/06/08 22:31:05 failed to get project metadata: Get "https://compute.s3nsapis.fr/compute/v1/projects/s3ns%3Aredacted-project?alt=json&prettyPrint=false": the configured universe domain ("s3nsapis.fr") does not match the universe domain found in the credentials ("googleapis.com"). If you haven't configured the universe domain explicitly, "googleapis.com" is the default.

[hashicorp-cla-app]

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