core/agent: soft context compact and cooperative cancel points

Shrink older tool dumps for model requests without mutating history;
check task cancellation between tool/model steps.

tests/test_agent/test_budget.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
-from plyngent.agent.budget import truncate_tool_result
+from plyngent.agent.budget import estimate_message_chars, truncate_tool_result
+from plyngent.lmproto.openai_compatible.model import ToolChatMessage, UserChatMessage
 
 
 def test_truncate_tool_result_short() -> None:
@@ -13,3 +14,9 @@ def test_truncate_tool_result_long() -> None:
     assert out.startswith("a" * 20)
     assert "truncated" in out
     assert "30" in out
+
+
+def test_estimate_message_chars() -> None:
+    assert estimate_message_chars(UserChatMessage(content="hello")) == 5  # noqa: PLR2004
+    tool = ToolChatMessage(content="abc", tool_call_id="id1")
+    assert estimate_message_chars(tool) == 6  # noqa: PLR2004