core[patch]: optimize trim_messages (#30327)

Refactored w/ Claude

Up to 20x speedup! (with theoretical max improvement of `O(n / log n)`)

libs/core/tests/unit_tests/messages/test_utils.py

@@ -455,6 +455,7 @@ def dummy_token_counter(messages: list[BaseMessage]) -> int:
 
 def test_trim_messages_partial_text_splitting() -> None:
     messages = [HumanMessage(content="This is a long message that needs trimming")]
+    messages_copy = [m.model_copy(deep=True) for m in messages]
 
     def count_characters(msgs: list[BaseMessage]) -> int:
         return sum(len(m.content) if isinstance(m.content, str) else 0 for m in msgs)
@@ -474,6 +475,7 @@ def test_trim_messages_partial_text_splitting() -> None:
 
     assert len(result) == 1
     assert result[0].content == "This is a "  # First 10 characters
+    assert messages == messages_copy
 
 
 def test_trim_messages_mixed_content_with_partial() -> None:
@@ -485,6 +487,7 @@ def test_trim_messages_mixed_content_with_partial() -> None:
             ]
         )
     ]
+    messages_copy = [m.model_copy(deep=True) for m in messages]
 
     # Count total length of all text parts
     def count_text_length(msgs: list[BaseMessage]) -> int:
@@ -509,6 +512,7 @@ def test_trim_messages_mixed_content_with_partial() -> None:
     assert len(result) == 1
     assert len(result[0].content) == 1
     assert result[0].content[0]["text"] == "First part of text."
+    assert messages == messages_copy
 
 
 def test_trim_messages_exact_token_boundary() -> None:
@@ -535,6 +539,7 @@ def test_trim_messages_exact_token_boundary() -> None:
         strategy="first",
     )
     assert len(result2) == 2
+    assert result2 == messages
 
 
 def test_trim_messages_start_on_with_allow_partial() -> None:
@@ -543,7 +548,7 @@ def test_trim_messages_start_on_with_allow_partial() -> None:
         AIMessage(content="AI response"),
         HumanMessage(content="Second human message"),
     ]
-
+    messages_copy = [m.model_copy(deep=True) for m in messages]
     result = trim_messages(
         messages,
         max_tokens=20,
@@ -555,6 +560,7 @@ def test_trim_messages_start_on_with_allow_partial() -> None:
 
     assert len(result) == 1
     assert result[0].content == "Second human message"
+    assert messages == messages_copy
 
 
 class FakeTokenCountingModel(FakeChatModel):