fix(auth): improve error reporting when token lookup fails

[Cris682]

Summary

This PR addresses #112 by making token lookup errors more actionable.
When token resolution fails, the final error is no longer only the generic authentication token not found ; it now includes concrete failure reasons when available (for example, gh not found in PATH or gh auth token execution failure).

Changes

• Added auth.TokenForHostWithError(host) :
  • Keeps the same token lookup order (env/config/gh),
  • Returns a detailed error when the gh fallback fails.
• Kept auth.TokenForHost(host) backward-compatible:
  • Signature unchanged,
  • Still returns ("", "default") when no token is found (or when lookup fails).
• Improved tokenFromGh error handling:
  • Switched to CombinedOutput() so stderr/output context is preserved in errors.
• Updated api.resolveOptions :
  • Uses TokenForHostWithError ,
  • If no token is found, includes the underlying cause when present.

Why

Previously, specific failures (e.g. missing gh binary) were masked by a generic “token not found” message, making debugging and support harder.

Tests

Added targeted tests:

• pkg/auth/auth_test.go
  • detailed error when gh is not in PATH ,
  • detailed error when GH_PATH points to an invalid executable.
• pkg/api/client_options_test.go
  • verifies resolveOptions surfaces both the generic message and the concrete cause ( could not find gh executable in PATH ).

Backward Compatibility

• No API breaking changes:
  • TokenForHost remains unchanged,
  • TokenForHostWithError is additive for callers that need detailed error context.

Related

• Closes TokenForHost "authentication token not found" message could be more informative #112