fix(cli): guard c-S-c key binding with try/except to prevent startup crash (#19895)

PR #19884 added @kb.add('c-S-c') unconditionally. prompt_toolkit raises
ValueError("Invalid key: c-S-c") during HermesCLI.__init__ on platforms
where this key spec is not recognised — the process exits before reaching
the prompt loop. Reported on macOS (#19894) and Linux (#19896) immediately
after #19884 landed.

Fix: wrap the registration in try/except ValueError so that startup
continues cleanly on any platform/version that rejects the spec. Where
the spec is accepted the binding is registered normally as a no-op,
allowing the terminal to handle Ctrl+Shift+C natively as before.

Fixes #19894
Fixes #19896

cli.py

@@ -10484,20 +10484,19 @@ class HermesCLI:
                     self._should_exit = True
                     event.app.exit()
 
-        @kb.add('c-S-c')  # Ctrl+Shift+C
-        def handle_ctrl_shift_c(event):
-            """Copy text to clipboard (terminal-native).
+        try:
+            @kb.add('c-S-c')  # Ctrl+Shift+C — no-op, let terminal handle native copy
+            def handle_ctrl_shift_c(event):
+                """No-op: let the terminal handle Ctrl+Shift+C natively.
 
-            This is a no-op at the application level. Terminal emulators
-            handle the actual copy operation when Ctrl+Shift+C is pressed.
-            This binding prevents Hermes from intercepting the keystroke
-            as an interrupt signal.
-
-            On macOS the standard copy shortcut is Cmd+C (no Hermes binding
-            needed). On Linux/Windows Ctrl+Shift+C is the conventional
-            terminal copy shortcut.
-            """
-            return  # No-op — let the terminal perform native copy
+                Wrapped in try/except because prompt_toolkit raises ValueError
+                ("Invalid key: c-S-c") on platforms where this key spec is not
+                recognised.  The binding is best-effort; if registration fails,
+                startup continues normally.
+                """
+                return
+        except ValueError:
+            pass  # prompt_toolkit on this platform/version doesn't support c-S-c
 
         @kb.add('c-q')  # Ctrl+Q
         def handle_ctrl_q(event):