fix(tui): repair shell-safety test scaffolding

Three independent test-scaffold defects blocked the unit_tests and
integration_tests gates on PR #6361's shell-safety wiring:

- features/steps/_tui_helpers.py: the mocked-shell helper patched
  cleveragents.tui.input.shell_exec.run_shell_command, but modes.py
  binds the symbol into its own namespace via `from ... import`. The
  patch was inert, and only the CLEVERAGENTS_ALLOW_DANGEROUS_SHELL=1
  gate kept the real `rm -rf /tmp` from running in the behave runner.
  Patch the use site (modes.run_shell_command) instead.

- src/cleveragents/tui/widgets/prompt.py: _FallbackPromptInput (used
  whenever Textual is mocked or unavailable) had no add_class /
  remove_class / has_class, so the new "prompt should be marked as
  dangerous" assertions raised AttributeError and the three new
  scenarios errored. The production path (_TextualPromptInput) already
  inherits these from textual.containers.Horizontal; the fallback now
  mirrors that contract via a small self._classes set.

- robot/tui_shell_safety.robot: Catenate's space-based argument
  separator collapses multi-space indentation, so the Python function
  bodies (warn_callback, deny) landed at column 0 and the helper
  scripts died with IndentationError before either assertion ran.
  Preserve the 4-space indent with ${SPACE * 4} markers.

ISSUES CLOSED: #6361

features/steps/_tui_helpers.py

@@ -19,13 +19,21 @@ def _submit_text(context, text: str) -> None:
 
 
 def _submit_text_with_mocked_shell(context, text: str, stdout: str = "mocked") -> None:
-    """Submit *text* while faking shell execution."""
+    """Submit *text* while faking shell execution.
+
+    The patch target is the use site in ``modes`` rather than the
+    definition site in ``shell_exec``: ``modes`` did
+    ``from cleveragents.tui.input.shell_exec import run_shell_command``,
+    binding the symbol into its own namespace; patching the source
+    module would leave that binding (and therefore the real
+    ``subprocess.run`` call) untouched.
+    """
 
     def fake_run(command: str, **_: Any) -> ShellResult:
         return ShellResult(command=command, exit_code=0, stdout=stdout, stderr="")
 
     with patch(
-        "cleveragents.tui.input.shell_exec.run_shell_command",
+        "cleveragents.tui.input.modes.run_shell_command",
         side_effect=fake_run,
     ):
         _submit_text(context, text)

robot/tui_shell_safety.robot

@@ -20,8 +20,8 @@ Shell Safety Service Blocks Denied Command
     ...    warnings: list[DangerousCommandWarning] = []
     ...
     ...    def warn_callback(warning: DangerousCommandWarning) -> bool:
-    ...        warnings.append(warning)
-    ...        return False
+    ...    ${SPACE * 4}warnings.append(warning)
+    ...    ${SPACE * 4}return False
     ...
     ...    router = InputModeRouter(lambda cmd: "handled", shell_safety=ShellSafetyService(warn_callback=warn_callback))
     ...    os.environ.pop("CLEVERAGENTS_ALLOW_DANGEROUS_SHELL", None)
@@ -47,8 +47,8 @@ Shell Confirm Callback Gates All Commands
     ...    counter = {"count": 0}
     ...
     ...    def deny(command: str) -> bool:
-    ...        counter["count"] += 1
-    ...        return False
+    ...    ${SPACE * 4}counter["count"] += 1
+    ...    ${SPACE * 4}return False
     ...
     ...    result = run_shell_command("chmod -R 777 /tmp/test-shell-safety", confirm_dangerous=deny)
     ...    assert counter["count"] == 1, f"Expected confirm to be invoked once, got {counter['count']}"

src/cleveragents/tui/widgets/prompt.py

@@ -248,12 +248,20 @@ class _TextualPromptInput(_PromptSymbolMixin, _HorizontalBase):
 
 
 class _FallbackPromptInput(_PromptSymbolMixin):
-    """Fallback prompt input used when Textual is unavailable."""
+    """Fallback prompt input used when Textual is unavailable.
+
+    Mirrors the Textual Widget CSS-class API (``add_class`` /
+    ``remove_class`` / ``has_class``) so callers that toggle styling
+    classes on the outer prompt (e.g. ``add_class("dangerous")`` from
+    the shell-safety surfacing path) work uniformly across production
+    and fallback/mock paths.
+    """
 
     def __init__(self, placeholder: str = "", **_: object) -> None:
         self.placeholder = placeholder
         self._input = cast(_MutableValueInput, _InputBase())
         self._current_symbol = _PROMPT_SYMBOLS[InputMode.NORMAL]
+        self._classes: set[str] = set()
         self._update_symbol(self._input.value)
 
     @property
@@ -270,6 +278,15 @@ class _FallbackPromptInput(_PromptSymbolMixin):
         if callable(focus):
             focus()
 
+    def add_class(self, name: str) -> None:
+        self._classes.add(name)
+
+    def remove_class(self, name: str) -> None:
+        self._classes.discard(name)
+
+    def has_class(self, name: str) -> bool:
+        return name in self._classes
+
     def _apply_symbol(self, symbol: str) -> None:
         self._current_symbol = symbol