fix(tools): isolate get_tool_definitions quiet_mode cache + dedup LCM injection (#17335)

Long-lived Gateway processes were sending duplicate tool names to
providers that enforce uniqueness:

  - DeepSeek:        'Tool names must be unique.'
  - Xiaomi MiMo:     'tools contains duplicate names: lcm_expand'
  - Moonshot/Kimi:   'function name lcm_grep is duplicated'

TUI was unaffected because TUI runs with quiet_mode=False and skips the
cache entirely.

Root cause (two layered bugs)
- model_tools.get_tool_definitions(quiet_mode=True) memoizes its result
  in _tool_defs_cache. The cache-hit path returned list(cached) (safe),
  but the FIRST uncached call stored and returned the SAME object.
  run_agent.py mutates self.tools (memory + LCM context-engine schemas)
  in-place, so the very first agent init in a Gateway process
  poisoned the cache, and every subsequent init appended LCM schemas
  again on top of the already-polluted list.
- run_agent.py's context-engine injection (lcm_grep / lcm_describe /
  lcm_expand) had no dedup, unlike the memory-tools injection right
  above it which already skips already-present names.

Fix (defense in depth, per the issue's suggested fix)
- model_tools.get_tool_definitions: on the uncached branch, cache the
  computed list but return list(result) to the caller. Same pattern as
  the cache-hit path.
- run_agent.py: build _existing_tool_names from self.tools and skip
  schemas whose names are already present, mirroring the memory-tools
  block. This also defends against plugin paths that may register the
  same schemas via ctx.register_tool().

Tests (tests/test_get_tool_definitions_cache_isolation.py)
- test_first_uncached_call_returns_fresh_list \u2014 pins the fix; without
  it, first-call alias caused all the symptoms.
- test_cache_hit_returns_fresh_list \u2014 pre-existing behavior stays.
- test_caller_mutation_does_not_poison_cache \u2014 simulates run_agent
  appending lcm_grep / lcm_expand to the returned list and asserts the
  next call doesn't see them.
- test_repeated_caller_mutation_does_not_accumulate \u2014 reproduces the
  long-lived Gateway accumulation pattern across 5 agent inits.
- test_non_quiet_mode_does_not_use_cache \u2014 sanity, explains why TUI
  was fine.

5/5 pass on the new file; 23/23 still pass on tests/test_model_tools.py.

tests/test_get_tool_definitions_cache_isolation.py

@@ -0,0 +1,94 @@
+"""Regression tests for issue #17335.
+
+The ``quiet_mode=True`` fast path in :func:`model_tools.get_tool_definitions`
+memoizes results to avoid re-walking the registry on every Gateway call. The
+cached object must NOT be aliased into callers' return values \u2014 long-lived
+Gateway processes mutate the returned list (``run_agent`` appends memory and
+LCM context-engine tool schemas to ``self.tools``), and a shared list would
+poison subsequent agent inits with duplicate tool names. Providers that
+enforce uniqueness (DeepSeek, Xiaomi MiMo, Moonshot/Kimi) then reject the
+API call with HTTP 400.
+
+These tests pin:
+- the cache-hit path returns a fresh list (existing #17098 behavior)
+- the first uncached call also returns a fresh list (the fix)
+- every call returns a list that is not the cached one, even after mutation
+"""
+from __future__ import annotations
+
+import pytest
+
+import model_tools
+
+
+@pytest.fixture(autouse=True)
+def _clear_cache():
+    """Each test starts with an empty quiet_mode cache."""
+    model_tools._tool_defs_cache.clear()
+    yield
+    model_tools._tool_defs_cache.clear()
+
+
+class TestQuietModeCacheIsolation:
+
+    def test_first_uncached_call_returns_fresh_list(self):
+        """The first quiet_mode call must not alias the cached object \u2014
+        otherwise a caller mutating the returned list mutates the cache."""
+        first = model_tools.get_tool_definitions(quiet_mode=True)
+        assert isinstance(first, list)
+        # Find the cached value to compare identity.
+        assert len(model_tools._tool_defs_cache) == 1
+        cached = next(iter(model_tools._tool_defs_cache.values()))
+        assert first is not cached, (
+            "issue #17335: first quiet_mode call returned the cached list "
+            "by reference \u2014 mutations will leak into subsequent calls."
+        )
+
+    def test_cache_hit_returns_fresh_list(self):
+        """The cache-hit path already returned a copy pre-fix; pin it."""
+        first = model_tools.get_tool_definitions(quiet_mode=True)
+        second = model_tools.get_tool_definitions(quiet_mode=True)
+        assert first is not second
+        cached = next(iter(model_tools._tool_defs_cache.values()))
+        assert second is not cached
+
+    def test_caller_mutation_does_not_poison_cache(self):
+        """Simulate run_agent appending LCM tool schemas to the returned
+        list. A second call must NOT see those appended entries."""
+        first = model_tools.get_tool_definitions(quiet_mode=True)
+        baseline_len = len(first)
+        # Caller mutates the returned list (this is what run_agent does
+        # when it injects memory + context-engine tool schemas).
+        first.append({"type": "function", "function": {"name": "lcm_grep"}})
+        first.append({"type": "function", "function": {"name": "lcm_expand"}})
+
+        second = model_tools.get_tool_definitions(quiet_mode=True)
+        # Length must match the original \u2014 cache pollution would make
+        # second 2 entries longer.
+        assert len(second) == baseline_len, (
+            f"issue #17335: cache was polluted by caller mutation. "
+            f"first len={baseline_len}, mutated len={len(first)}, "
+            f"second-call len={len(second)} \u2014 expected {baseline_len}."
+        )
+        names = [t.get("function", {}).get("name") for t in second]
+        assert "lcm_grep" not in names
+        assert "lcm_expand" not in names
+
+    def test_repeated_caller_mutation_does_not_accumulate(self):
+        """The original Gateway symptom: every agent init in a long-lived
+        process appends LCM schemas, accumulating duplicates over time."""
+        baseline = len(model_tools.get_tool_definitions(quiet_mode=True))
+        for _ in range(5):
+            tools = model_tools.get_tool_definitions(quiet_mode=True)
+            tools.append({"type": "function", "function": {"name": "lcm_grep"}})
+        final = model_tools.get_tool_definitions(quiet_mode=True)
+        assert len(final) == baseline, (
+            f"Cache accumulated mutations across {5} agent inits: "
+            f"baseline={baseline}, final={len(final)}."
+        )
+
+    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."""
+        model_tools.get_tool_definitions(quiet_mode=False)
+        assert len(model_tools._tool_defs_cache) == 0