fix: report Splunk auth failures with ActionFailure

[mfasil-tcs]

Bug Fixes

• Improved Splunk authentication failure handling for REST API responses.
• Added explicit handling for 401 and 403 responses so the app now reports whether authentication failed using the Splunk API token or username/password.
• Updated connectivity-test flow to preserve ActionFailure messages instead of converting known authentication failures into generic connectivity/runtime errors.
• Updated retry and server-check paths so known action failures are surfaced consistently across actions that use the shared Splunk helper.

Manual Documentation

Have you made any changes that should be documented in manual_readme_content.md?

The following changes require documentation in manual_readme_content.md:

• New, updated, or removed REST handlers
• New, updated, or removed authentication methods, especially complex methods like OAuth
• Compatibility considerations with respect to deployment types (e.g. actions that cannot be run on cloud or an automation broker)

• I have verified that manual documentation has been updated where appropriate

Other information

• Regenerated generated documentation and NOTICE through pre-commit hooks.