opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

tests/run_agent/test_partial_stream_finish_reason.py

@@ -136,101 +136,6 @@ class TestPartialStreamStubFinishReason:
         assert "write_file" in content
 
 
-# ── Clean stream-end mid-tool-call (no exception, no finish_reason) ─────────
-
-class TestCleanStreamEndMidToolCall:
-    """The upstream closes the SSE stream cleanly after delivering a tool
-    name + the opening '{' of its arguments — NO exception, NO finish_reason,
-    NO [DONE].  Observed live on NVIDIA Nemotron Ultra via the Nous dedicated
-    endpoint: it stalls/drops during large tool-arg generation.
-
-    The mock-builder must NOT stamp this as finish_reason='length' (which
-    routes it through the max_tokens-boost truncation path and finally
-    reports the misleading 'Response truncated due to output length limit').
-    It must route through the partial-stream-stub path so the loop reports
-    an honest mid-tool-call drop and asks the model to chunk its output.
-    """
-
-    @patch("run_agent.AIAgent._create_request_openai_client")
-    @patch("run_agent.AIAgent._close_request_openai_client")
-    def test_no_finish_reason_partial_tool_args_routes_to_stub(
-        self, _mock_close, mock_create, monkeypatch,
-    ):
-        def _clean_ending_stream():
-            # Reasoning + tool name + the lone opening brace, then the
-            # generator simply RETURNS (StopIteration) — no raise, no
-            # finish_reason chunk, no [DONE].
-            yield _make_stream_chunk(content="\n")
-            yield _make_stream_chunk(tool_calls=[
-                _make_tool_call_delta(index=0, tc_id="call_x", name="execute_code"),
-            ])
-            yield _make_stream_chunk(tool_calls=[
-                _make_tool_call_delta(index=0, arguments="{"),
-            ])
-            # falls off the end — clean close, no terminator
-
-        mock_client = MagicMock()
-        mock_client.chat.completions.create.side_effect = (
-            lambda *a, **kw: _clean_ending_stream()
-        )
-        mock_create.return_value = mock_client
-
-        agent = _make_agent()
-        agent._fire_stream_delta = lambda text: None
-
-        response = agent._interruptible_streaming_api_call({})
-
-        assert response.id == PARTIAL_STREAM_STUB_ID, (
-            "A clean stream-end mid tool-call (no finish_reason) must be "
-            "tagged as a partial-stream stub, not a 'stream-<uuid>' "
-            "truncation — otherwise the loop reports the false 'output "
-            "length limit' error."
-        )
-        assert response.choices[0].finish_reason == FINISH_REASON_LENGTH
-        assert response.choices[0].message.tool_calls is None, (
-            "Incomplete tool args must never auto-execute."
-        )
-        assert getattr(response, "_dropped_tool_names", None) == ["execute_code"]
-
-    @patch("run_agent.AIAgent._create_request_openai_client")
-    @patch("run_agent.AIAgent._close_request_openai_client")
-    def test_real_length_truncation_still_uses_uuid_id(
-        self, _mock_close, mock_create, monkeypatch,
-    ):
-        """Control: when the provider DOES send finish_reason='length' with
-        partial tool args, it is a genuine output cap — keep the existing
-        non-stub behaviour (boost max_tokens and retry)."""
-
-        def _capped_stream():
-            yield _make_stream_chunk(tool_calls=[
-                _make_tool_call_delta(index=0, tc_id="call_y", name="execute_code"),
-            ])
-            yield _make_stream_chunk(tool_calls=[
-                _make_tool_call_delta(index=0, arguments="{"),
-            ])
-            # Provider explicitly reports the output cap.
-            yield _make_stream_chunk(finish_reason="length")
-
-        mock_client = MagicMock()
-        mock_client.chat.completions.create.side_effect = (
-            lambda *a, **kw: _capped_stream()
-        )
-        mock_create.return_value = mock_client
-
-        agent = _make_agent()
-        agent._fire_stream_delta = lambda text: None
-
-        response = agent._interruptible_streaming_api_call({})
-
-        assert response.id != PARTIAL_STREAM_STUB_ID, (
-            "A provider-reported finish_reason='length' is a real output cap "
-            "and must keep the existing truncation path, not the stream-drop "
-            "stub path."
-        )
-        assert response.id.startswith("stream-")
-        assert response.choices[0].finish_reason == FINISH_REASON_LENGTH
-
-
 # ── Length-continuation prompt branching ──────────────────────────────────
 
 class TestLengthContinuationPromptBranching: