fix(cli): bracketed-paste timeout prevents permanent input freeze (#16263)

When the terminal drops the ESC[201~ end mark during a bracketed paste
(terminal race, torn write, SSH glitch, macOS sleep/wake), prompt_toolkit's
Vt100Parser keeps buffering all later input in _paste_buffer forever. From
the user's perspective, the CLI appears frozen — the only recovery was
closing the tab/session.

This patch monkey-patches Vt100Parser.feed() so that bracketed-paste mode
flushes buffered content as a normal BracketedPaste event after 2 seconds
without an end marker, then restores normal parsing.

Includes 8 regression tests covering normal paste, timeout recovery,
torn end marks, and edge cases.

Surgical reapply of PR #27518. Original branch was many months stale
(1193 files / 172k LOC of unrelated reverts); the substantive ~77 LOC
patch in cli.py plus the new 157-line test file were reapplied onto
current main with the contributor's authorship preserved via --author.

cli.py

@@ -2360,6 +2360,89 @@ def _strip_leaked_bracketed_paste_wrappers(text: str) -> str:
     return text
 
 
+def _apply_bracketed_paste_timeout_patch() -> None:
+    """Patch prompt_toolkit to recover from torn bracketed-paste sequences.
+
+    prompt_toolkit's ``Vt100Parser.feed()`` buffers all input while waiting
+    for the ESC[201~ end mark.  If a terminal drops that end mark (terminal
+    race, torn write, SSH glitch, macOS sleep/wake), input appears frozen
+    forever — the only recovery used to be killing the tab.
+
+    This patch wraps ``Vt100Parser.feed`` so that bracketed-paste mode
+    flushes buffered content as a normal ``BracketedPaste`` event after
+    ``_BP_TIMEOUT_S`` seconds without an end marker, then resumes normal
+    parsing.  See upstream issue #16263.
+
+    The patch is idempotent — repeated calls are no-ops via the
+    ``_hermes_bp_timeout_patched`` sentinel on the module.
+    """
+    try:
+        import prompt_toolkit.input.vt100_parser as _vt100_mod
+        from prompt_toolkit.keys import Keys as _PtKeys
+        from prompt_toolkit.key_binding.key_processor import KeyPress as _PtKeyPress
+
+        if getattr(_vt100_mod, "_hermes_bp_timeout_patched", False):
+            return
+
+        _BP_TIMEOUT_S = 2.0  # max time to wait for ESC[201~ before flushing
+
+        def _patched_vt100_feed(self_parser, data: str) -> None:
+            if self_parser._in_bracketed_paste:
+                self_parser._paste_buffer += data
+                end_mark = "\x1b[201~"
+
+                if end_mark in self_parser._paste_buffer:
+                    end_index = self_parser._paste_buffer.index(end_mark)
+                    paste_content = self_parser._paste_buffer[:end_index]
+                    self_parser.feed_key_callback(
+                        _PtKeyPress(_PtKeys.BracketedPaste, paste_content)
+                    )
+                    self_parser._in_bracketed_paste = False
+                    remaining = self_parser._paste_buffer[
+                        end_index + len(end_mark):
+                    ]
+                    self_parser._paste_buffer = ""
+                    self_parser._hermes_bp_start = None
+                    if remaining:
+                        _patched_vt100_feed(self_parser, remaining)
+                else:
+                    bp_start = getattr(self_parser, "_hermes_bp_start", None)
+                    now = time.monotonic()
+                    if bp_start is None:
+                        self_parser._hermes_bp_start = now
+                    elif now - bp_start > _BP_TIMEOUT_S:
+                        paste_content = self_parser._paste_buffer
+                        self_parser._in_bracketed_paste = False
+                        self_parser._paste_buffer = ""
+                        self_parser._hermes_bp_start = None
+                        if paste_content:
+                            self_parser.feed_key_callback(
+                                _PtKeyPress(_PtKeys.BracketedPaste, paste_content)
+                            )
+                            logger.warning(
+                                "Bracketed-paste timeout (%.1fs) — flushed %d bytes "
+                                "without end mark. Terminal may have dropped ESC[201~ "
+                                "(see #16263).",
+                                now - bp_start,
+                                len(paste_content),
+                            )
+            else:
+                # Normal mode — re-inline prompt_toolkit's normal feed path.
+                # Calling the original feed here would double-buffer after the
+                # bracketed-paste entry transition.
+                for i, c in enumerate(data):
+                    if self_parser._in_bracketed_paste:
+                        _patched_vt100_feed(self_parser, data[i:])
+                        break
+                    self_parser._input_parser.send(c)
+
+        _vt100_mod.Vt100Parser.feed = _patched_vt100_feed
+        _vt100_mod._hermes_bp_timeout_patched = True
+        logger.debug("Applied Vt100Parser bracketed-paste timeout patch (#16263)")
+    except Exception as exc:  # noqa: BLE001 — defensive: never break startup
+        logger.debug("Bracketed-paste timeout patch skipped: %s", exc)
+
+
 # Cursor Position Report (CPR / DSR) response, format ``ESC[<row>;<col>R``.
 # prompt_toolkit's _on_resize() + renderer send ``ESC[6n`` queries to the
 # terminal; under resize storms or tab switches the terminal's reply can
@@ -14151,6 +14234,10 @@ class HermesCLI:
         except Exception:
             pass
 
+        # Apply bracketed-paste timeout recovery so torn ESC[201~ end marks
+        # don't permanently freeze the input (issue #16263). Idempotent.
+        _apply_bracketed_paste_timeout_patch()
+
         _original_on_resize = app._on_resize
 
         def _resize_clear_ghosts():