Add basic validation on credentials when instantiating clients

[nikola-jokic]

Fixes #100

[Copilot]

Pull request overview

This PR addresses issue #100 by adding early credential validation when creating a scaleset client, preventing later panics when neither a GitHub App credential set nor a Personal Access Token (PAT) is provided.

Changes:

• Added actionsAuth.validate() and enforced credential validation during newClient() construction.
• Added unit tests for NewClientWithPersonalAccessToken and NewClientWithGitHubApp, including error cases for missing credentials.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
client.go	Adds credential validation in actionsAuth and invokes it during client creation to fail fast on missing/invalid auth.
client_test.go	Adds coverage for successful client construction and expected errors when credentials are missing/invalid.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.