revert(mcp): drop SendMessage from iframe apply on_error

Reconsidered: the existing UpdateContext on the on_error chain already
pushes the failure into the agent's model context — the agent reacts on
its next turn, mirroring the on_success path's UpdateContext(content=
RESULT). Adding a SendMessage doubles up: it puts a chat-visible
"Apply failed: ..." line that the agent then has to deduplicate against
the same content arriving via UpdateContext, AND adds chat noise on the
common-rare error path.

Keep the existing rail unchanged:
- SetState/ShowToast → iframe feedback (toast for user, state for UI)
- UpdateContext → agent receives the error reason

The Bug #1 datetime fix (this PR) eliminates the load-bearing trigger
for the SRAM-session Bug #4 — naive datetimes will no longer 422
silently. If iframe-error visibility remains an issue on a different
class of failure, address it separately by investigating WHY the
existing toast / UpdateContext aren't reaching their audience rather
than layering SendMessage on top.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dougborg

katana_mcp_server/src/katana_mcp/tools/prefab_ui.py

@@ -332,13 +332,6 @@ def _build_apply_action_direct(
                 SetState("pending", False),
                 SetState("error", "{{ $error }}"),
                 ShowToast("{{ $error }}", variant="error"),
-                # Mirror the cancel-button rail (line ~355): send a chat-
-                # visible message so the user sees the failure in chat
-                # instead of relying on the iframe toast (which is
-                # easy to miss / may not render in some hosts). The
-                # ``UpdateContext`` below is the agent-visible channel —
-                # this ``SendMessage`` is the user-visible one.
-                SendMessage("Apply failed: {{ $error }}"),
                 UpdateContext(content="Apply failed: {{ $error }}"),
             ],
         ),

katana_mcp_server/tests/test_prefab_ui.py

@@ -1047,23 +1047,6 @@ def test_po_preview_direct_apply_handles_error(self):
         toasts = [a for a in on_error if a.get("action") == "showToast"]
         assert len(toasts) == 1, f"Expected one error toast; got {toasts!r}"
 
-        # Bug #4 (2026-05-12 SRAM session): the iframe toast alone was
-        # invisible to the user during a live session where 9 confirm
-        # clicks silently failed against a Katana 422. The cancel-button
-        # rail uses SendMessage to surface back to chat; mirror that on
-        # error so users see a chat-visible message, not just an
-        # in-iframe toast.
-        send_messages = [a for a in on_error if a.get("action") == "sendMessage"]
-        assert len(send_messages) == 1, (
-            f"Expected one SendMessage on error so the failure surfaces in "
-            f"chat (not just the iframe toast); got {send_messages!r}"
-        )
-        send_content = send_messages[0].get("content", "")
-        assert "$error" in send_content, (
-            f"SendMessage on error must inline the $error reason so the user "
-            f"sees what went wrong; got {send_messages[0]!r}"
-        )
-
     def test_po_preview_direct_apply_double_click_is_guarded(self):
         """Confirm button binds ``pending`` so a double-click cannot fire
         two applies. Ensures (1) on_click sets pending=True before