tests: pin ink engine in _make_tui_argv npm-bootstrap tests (post-merge semantic fix)

Main's rewritten test_tui_npm_install.py tests call _make_tui_argv expecting
the Ink/npm flow unconditionally; with the dual-engine dispatch merged in,
_resolve_tui_engine() auto-selects opentui whenever ui-opentui/dist is built
in the repo, routing the call away from the path under test (first subprocess
became 'node --version' instead of 'npm run build'). Pin the engine to ink
via an autouse fixture, mirroring the existing pinning precedent in
test_tui_resume_flow.py.

tests/test_get_tool_definitions_cache_isolation.py

@@ -87,6 +87,27 @@ class TestQuietModeCacheIsolation:
             f"baseline={baseline}, final={len(final)}."
         )
 
+    def test_cache_bounded_by_eviction(self):
+        """The cache evicts the oldest entry when it reaches the cap,
+        keeping the cache bounded instead of growing unbounded over a
+        long-lived Gateway's lifetime (#19251)."""
+        cap = model_tools._TOOL_DEFS_CACHE_MAX
+        # Fill cache to the cap with distinct keys by varying enabled_toolsets.
+        for i in range(cap):
+            model_tools.get_tool_definitions(
+                enabled_toolsets=[f"fake_toolset_{i}"], quiet_mode=True,
+            )
+        assert len(model_tools._tool_defs_cache) == cap
+
+        # Adding one more must evict the oldest, not clear everything and
+        # not grow past the cap.
+        model_tools.get_tool_definitions(
+            enabled_toolsets=["fake_toolset_overflow"], quiet_mode=True,
+        )
+        assert len(model_tools._tool_defs_cache) == cap, (
+            "Eviction should keep the cache at the cap, not clear it or grow"
+        )
+
     def test_non_quiet_mode_does_not_use_cache(self):
         """Sanity: quiet_mode=False (TUI path) skips the cache entirely \u2014
         explains why the bug only hit Gateway."""