fix(tui-gateway): dispatch slow RPC handlers on a thread pool (#12546)

The stdin-read loop in entry.py calls handle_request() inline, so the
five handlers that can block for seconds to minutes
(slash.exec, cli.exec, shell.exec, session.resume, session.branch)
freeze the dispatcher. While one is running, any inbound RPC —
notably approval.respond and session.interrupt — sits unread in the
pipe buffer and lands only after the slow handler returns.

Route only those five onto a small ThreadPoolExecutor; every other
handler stays on the main thread so the fast-path ordering is
unchanged and the audit surface stays small. write_json is already
_stdout_lock-guarded, so concurrent response writes are safe. Pool
size defaults to 4 (overridable via HERMES_TUI_RPC_POOL_WORKERS).

- add _LONG_HANDLERS set + ThreadPoolExecutor + atexit shutdown
- new dispatch(req) function: pool for long handlers, inline for rest
- _run_and_emit wraps pool work in a try/except so a misbehaving
  handler still surfaces as a JSON-RPC error instead of silently
  dying in a worker
- entry.py swaps handle_request → dispatch
- 5 new tests: sync path still inline, long handlers emit via stdout,
  fast handler not blocked behind slow one, handler exceptions map to
  error responses, non-long methods always take the sync path

Manual repro confirms the fix: shell.exec(sleep 3) + terminal.resize
sent back-to-back now returns the resize response at t=0s while the
sleep finishes independently at t=3s. Before, both landed together
at t=3s.

Fixes #12546.

tests/tui_gateway/test_protocol.py

@@ -4,6 +4,7 @@ import io
 import json
 import sys
 import threading
+import time
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -432,3 +433,81 @@ def test_command_dispatch_returns_skill_payload(server):
     assert result["type"] == "skill"
     assert result["message"] == fake_msg
     assert result["name"] == "hermes-agent-dev"
+
+
+# ── dispatch(): pool routing for long handlers (#12546) ──────────────
+
+
+def test_dispatch_runs_short_handlers_inline(server):
+    """Non-long handlers return their response synchronously from dispatch()."""
+    server._methods["fast.ping"] = lambda rid, params: server._ok(rid, {"pong": True})
+
+    resp = server.dispatch({"id": "r1", "method": "fast.ping", "params": {}})
+
+    assert resp == {"jsonrpc": "2.0", "id": "r1", "result": {"pong": True}}
+
+
+def test_dispatch_offloads_long_handlers_and_emits_via_stdout(capture):
+    """Long handlers run on the pool and write their response via write_json."""
+    server, buf = capture
+    server._methods["slash.exec"] = lambda rid, params: server._ok(rid, {"output": "hi"})
+
+    resp = server.dispatch({"id": "r2", "method": "slash.exec", "params": {}})
+    assert resp is None
+
+    for _ in range(50):
+        if buf.getvalue():
+            break
+        time.sleep(0.01)
+
+    written = json.loads(buf.getvalue())
+    assert written == {"jsonrpc": "2.0", "id": "r2", "result": {"output": "hi"}}
+
+
+def test_dispatch_long_handler_does_not_block_fast_handler(server):
+    """A slow long handler must not prevent a concurrent fast handler from completing."""
+    released = threading.Event()
+    server._methods["slash.exec"] = lambda rid, params: (released.wait(timeout=5), server._ok(rid, {"done": True}))[1]
+    server._methods["fast.ping"] = lambda rid, params: server._ok(rid, {"pong": True})
+
+    t0 = time.monotonic()
+    assert server.dispatch({"id": "slow", "method": "slash.exec", "params": {}}) is None
+
+    fast_resp = server.dispatch({"id": "fast", "method": "fast.ping", "params": {}})
+    fast_elapsed = time.monotonic() - t0
+
+    assert fast_resp["result"] == {"pong": True}
+    assert fast_elapsed < 0.5, f"fast handler blocked for {fast_elapsed:.2f}s behind slow handler"
+
+    released.set()
+
+
+def test_dispatch_long_handler_exception_produces_error_response(capture):
+    """An exception inside a pool-dispatched handler still yields a JSON-RPC error."""
+    server, buf = capture
+
+    def boom(rid, params):
+        raise RuntimeError("kaboom")
+
+    server._methods["slash.exec"] = boom
+
+    server.dispatch({"id": "r3", "method": "slash.exec", "params": {}})
+
+    for _ in range(50):
+        if buf.getvalue():
+            break
+        time.sleep(0.01)
+
+    written = json.loads(buf.getvalue())
+    assert written["id"] == "r3"
+    assert written["error"]["code"] == -32000
+    assert "kaboom" in written["error"]["message"]
+
+
+def test_dispatch_unknown_long_method_still_goes_inline(server):
+    """Method name not in _LONG_HANDLERS takes the sync path even if handler is slow."""
+    server._methods["some.method"] = lambda rid, params: server._ok(rid, {"ok": True})
+
+    resp = server.dispatch({"id": "r4", "method": "some.method", "params": {}})
+
+    assert resp["result"] == {"ok": True}