fix: fully drain terminal paste writes to PTY

- Make the host PTY master non-blocking and wait for write readiness.
- Retry partial writes so a large bracketed paste always delivers its closing
  marker instead of leaving pi in paste mode.
- Add regression coverage for partial PTY writes and resolve diagnostics.

Quality gates: ruff, mypy, focused pytest (3 passed)

apps/api/src/services/terminal/terminal_session.py

@@ -19,6 +19,8 @@ from typing import Any
 
 logger = logging.getLogger(__name__)
 
+_session_counters_by_instance: dict[str, int] = {}
+
 
 class TerminalSession:
     """Manages a single terminal session with event-driven PTY I/O.
@@ -46,9 +48,6 @@ class TerminalSession:
     # Max WebSocket frame size
     MAX_FRAME_SIZE = 64 * 1024
 
-    # Session number counter per instance_id for auto-naming
-    _instance_counters: dict[str, int] = {}
-
     def __init__(
         self,
         session_id: str,
@@ -99,11 +98,11 @@ class TerminalSession:
         # Ack timeout fallback
         self._ack_timeout_handle: asyncio.TimerHandle | None = None
 
-    @classmethod
-    def _generate_name(cls, instance_id: str) -> str:
+    @staticmethod
+    def _generate_name(instance_id: str) -> str:
         """Generate an auto-incremented session name for the instance."""
-        count = cls._instance_counters.get(instance_id, 0) + 1
-        cls._instance_counters[instance_id] = count
+        count = _session_counters_by_instance.get(instance_id, 0) + 1
+        _session_counters_by_instance[instance_id] = count
         return f"Session {count}"
 
     async def start(self, startup_command: str | None = None) -> None:
@@ -121,8 +120,12 @@ class TerminalSession:
         if self.container_user:
             exec_cmd.extend(["--user", self.container_user])
 
-        # Create a pseudo-terminal on the host
+        # Create a pseudo-terminal on the host. The master must be
+        # non-blocking: a browser paste can be larger than the PTY input
+        # buffer, and write_input() drains it asynchronously without dropping
+        # the closing bracketed-paste marker.
         self._master_fd, slave_fd = pty.openpty()
+        os.set_blocking(self._master_fd, False)
 
         # Put the host PTY into raw mode so it behaves as a pass-through
         # pipe. openpty() leaves the slave in canonical mode by default,
@@ -204,6 +207,9 @@ class TerminalSession:
 
         try:
             data = os.read(self._master_fd, 4096)
+        except BlockingIOError:
+            # The readiness notification raced with another callback.
+            return
         except (OSError, IOError) as exc:
             logger.debug("PTY read error for session %s: %s", self.session_id, exc)
             self._handle_eof()
@@ -343,12 +349,42 @@ class TerminalSession:
                 pass
         logger.info("Session %s EOF handled, websockets closed", self.session_id)
 
+    async def _wait_for_write_ready(self, fd: int) -> None:
+        """Wait until a non-blocking PTY master can accept more input."""
+        loop = asyncio.get_running_loop()
+        writable = loop.create_future()
+
+        def mark_writable() -> None:
+            if not writable.done():
+                writable.set_result(None)
+
+        loop.add_writer(fd, mark_writable)
+        try:
+            await writable
+        finally:
+            loop.remove_writer(fd)
+
     async def write_input(self, data: bytes) -> None:
-        """Write input to the PTY master."""
+        """Write all terminal input bytes to the PTY master in order."""
         if self._master_fd is None or self._closed:
             return
+
+        fd = self._master_fd
+        remaining = memoryview(data)
         try:
-            os.write(self._master_fd, data)
+            while remaining and not self._closed and self._master_fd == fd:
+                try:
+                    written = os.write(fd, remaining)
+                except BlockingIOError:
+                    await self._wait_for_write_ready(fd)
+                    continue
+
+                if written == 0:
+                    await self._wait_for_write_ready(fd)
+                    continue
+
+                remaining = remaining[written:]
+
             self.last_activity = time.time()
         except (OSError, IOError) as exc:
             logger.debug("PTY write error for session %s: %s", self.session_id, exc)