User-facing validation errors currently display full Python tracebacks,
creating poor UX for common scenarios like invalid molecule.yml
configuration or missing scenarios.

Fixes #4532

Replace `raise ImmediateExit()` calls with `sysexit_with_message()` for
user-facing validation errors, simplifying error handling by using
direct `sys.exit()` instead of exception propagation.

Source code:
- Replace `ImmediateExit` exceptions with `sysexit_with_message()` calls
in validation functions
- Maintain existing logging behavior (`logger.critical`) and exit codes
- Preserve `ImmediateExit` handling in Click decorators for integration
compatibility
- Update docstrings to reflect direct exit behavior

Tests:
- Update tests to catch `SystemExit` instead of `ImmediateExit`
- Modify test assertions to verify logged messages via `caplog` instead
of exception attributes

This change moves from an exception-based exit pattern to a direct exit
pattern for user-facing validation errors. The `sysexit_with_message()`
function logs the error message using `logger.critical()` and calls
`sys.exit()` directly, providing the same user experience with cleaner
code flow.

Functions affected include config validation, scenario verification,
host selection validation, and dependency installation error handling.

Before:
```
$ molecule login
CRITICAL Failed to validate /path/to/molecule.yml

<full Python traceback>
molecule.exceptions.MoleculeError
```

After:
```
$ molecule login
CRITICAL Failed to validate /path/to/molecule.yml
```

This provides clean error messages without tracebacks while maintaining
the same logging behavior and exit codes.

---------

Co-authored-by: Kate Case <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Copilot <[email protected]>