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/tests/unit/test_terminal_session.py

@@ -1,9 +1,9 @@
-"""Unit tests for TerminalSession docker exec invocation."""
+"""Unit tests for TerminalSession PTY handling."""
 
 import uuid
 from unittest.mock import AsyncMock, patch
 
-import pytest
+import pytest  # pyright: ignore[reportMissingImports]
 
 from src.services.terminal.terminal_session import TerminalSession
 
@@ -19,17 +19,20 @@ async def test_start_passes_container_user_to_docker_exec() -> None:
         container_user="dev",
     )
 
-    with patch(
-        "src.services.terminal.terminal_session.pty.openpty",
-        return_value=(1, 2),
+    with (
+        patch(
+            "src.services.terminal.terminal_session.pty.openpty",
+            return_value=(1, 2),
+        ),
+        patch("src.services.terminal.terminal_session.os.set_blocking") as set_blocking,
+        patch("src.services.terminal.terminal_session.tty.setraw"),
+        patch(
+            "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
+            new=AsyncMock(),
+        ) as mock_exec,
+        patch("src.services.terminal.terminal_session.os.close"),
     ):
-        with patch("src.services.terminal.terminal_session.tty.setraw"):
-            with patch(
-                "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
-                new=AsyncMock(),
-            ) as mock_exec:
-                with patch("src.services.terminal.terminal_session.os.close"):
-                    await session.start()
+        await session.start()
 
     args, _kwargs = mock_exec.call_args
     assert "docker" in args
@@ -38,6 +41,7 @@ async def test_start_passes_container_user_to_docker_exec() -> None:
     user_index = args.index("--user")
     assert args[user_index + 1] == "dev"
     assert "container-123" in args
+    set_blocking.assert_called_once_with(1, False)
 
 
 @pytest.mark.unit
@@ -50,17 +54,44 @@ async def test_start_omits_user_when_not_configured() -> None:
         container_id="container-123",
     )
 
-    with patch(
-        "src.services.terminal.terminal_session.pty.openpty",
-        return_value=(1, 2),
+    with (
+        patch(
+            "src.services.terminal.terminal_session.pty.openpty",
+            return_value=(1, 2),
+        ),
+        patch("src.services.terminal.terminal_session.os.set_blocking"),
+        patch("src.services.terminal.terminal_session.tty.setraw"),
+        patch(
+            "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
+            new=AsyncMock(),
+        ) as mock_exec,
+        patch("src.services.terminal.terminal_session.os.close"),
     ):
-        with patch("src.services.terminal.terminal_session.tty.setraw"):
-            with patch(
-                "src.services.terminal.terminal_session.asyncio.create_subprocess_exec",
-                new=AsyncMock(),
-            ) as mock_exec:
-                with patch("src.services.terminal.terminal_session.os.close"):
-                    await session.start()
+        await session.start()
 
     args, _kwargs = mock_exec.call_args
     assert "--user" not in args
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_write_input_retries_partial_pty_writes() -> None:
+    """A large paste is fully written even when the PTY accepts it in chunks."""
+    session = TerminalSession(
+        session_id=str(uuid.uuid4()),
+        instance_id=uuid.uuid4(),
+        container_id="container-123",
+    )
+    session._master_fd = 42
+
+    with patch(
+        "src.services.terminal.terminal_session.os.write",
+        side_effect=[2, 2, 1],
+    ) as write:
+        await session.write_input(b"hello")
+
+    assert [bytes(call.args[1]) for call in write.call_args_list] == [
+        b"hello",
+        b"llo",
+        b"o",
+    ]