Improve error message for credential failures in non-interactive environments

[Yashagarwal9798]

Fixes #59341

Summary

This PR improves error reporting when SSH or User/Pass authentication fails in non-interactive environments (e.g., CI/CD pipelines). It replaces the misleading GitError: user cancelled credential request with a detailed error message offering actionable solutions.

Changes

• stdlib/LibGit2/src/callbacks.jl: Introduced exhausted_abort_noninteractive(). This helper is now called by authenticate_ssh and authenticate_userpass when credentials fail and allow_prompt=false.
• stdlib/LibGit2/test/libgit2-tests.jl: Updated 5 existing tests to verify the new, descriptive error message is thrown.

New Error Output

Instead of "user cancelled", users will now see:

Authentication failed. The remote server requires authentication, but no valid credentials were provided and the environment is non-interactive (prompting is disabled).

To provide SSH credentials, you can:

• Set the SSH_KEY_PATH environment variable...
• Ensure ssh-agent is running...
• [etc]

Testing

• Verified that interactive mode behavior remains unchanged.
• Confirmed that setting allow_prompt=false triggers the new error message instead of the generic GitError.

[DilumAluthge]

@Yashagarwal9798 Just to check, did you use any generative AI tools to write the code or the PR description?

[Yashagarwal9798]

Yes, I used generative AI to help refine the implementation and description after performing a detailed manual analysis of the issue. I have carefully reviewed all the code to ensure it meets Julia's standards and correctly addresses the problem

[DilumAluthge]

Please disclose the use of generative AI when you open the PR. This is a requirement of the Julia repo, documented in CONTRIBUTING.md.

I notice that you were also reminded of this requirement previously: #60640 (comment)

[Yashagarwal9798]

Understood. I apologize for the oversight and will ensure all future PRs include the required AI disclosure as per the contribution guidelines.