test(cli): cover UsageError path in main() and drop redundant handler

The mro-based UsageError check inside the Exception block already
catches BadParameter (it inherits from UsageError) — the separate
typer.BadParameter handler was redundant.

Added an in-process Behave step that calls main() directly and
captures err_console output, plus a scenario that runs
`plan use --no-such-flag` to cover the UsageError branch (subprocess
steps do not count toward unit-test coverage).

features/main_error_paths.feature

@@ -35,3 +35,9 @@ Feature: CLI main() error handling paths
   Scenario: _print_basic_help prints without error
     When I call the main _print_basic_help
     Then the main _print_basic_help completes ok
+
+  @coverage
+  Scenario: unknown option triggers UsageError handler with exit code 2
+    When I call main with arguments ["plan", "use", "--no-such-flag", "x"]
+    Then the main cli exit code should be 2
+    And the main cli output contains "No such option"