stop echoing credential contents in GetCredentials error

[naruto-lgtm]

GetCredentials resolves the provider credentials argument (a service account key, private key included) and, when the google auth library cannot load it, returns unable to parse credentials from '<contents>': <err> in transport/config.go. Any credentials value that is valid JSON but not loadable reaches that branch, so a mistyped or truncated key file has its full contents surfaced in a provider error that Terraform prints to the terminal and to CI logs, none of which is redacted.

Drop the credential contents from both error strings so only the underlying parse error is shown, which is what every other error path in this function already does. Keeping the redaction in the callee means no caller has to remember to scrub credentials before surfacing the failure.

provider: fixed an issue where an invalid `credentials` value could be echoed back in the error message when it failed to parse

[github-actions]

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