fix(tests): update recent history truncation to use token limits

tests/agent/test_context_prompt_cache.py

@@ -222,18 +222,23 @@ def test_recent_history_capped_at_max(tmp_path) -> None:
     assert f"entry-{builder._MAX_RECENT_HISTORY + 19}" in prompt
 
 
-def test_recent_history_truncated_at_max_chars(tmp_path) -> None:
-    """Recent History section must be truncated at _MAX_HISTORY_CHARS."""
+def test_recent_history_truncated_at_max_tokens(tmp_path) -> None:
+    """Recent History section must be truncated to _MAX_HISTORY_TOKENS."""
+    import tiktoken
+
     workspace = _make_workspace(tmp_path)
     builder = ContextBuilder(workspace)
 
-    big_entry = "x" * (builder._MAX_HISTORY_CHARS + 5_000)
+    big_entry = "word " * (builder._MAX_HISTORY_TOKENS + 5_000)
     builder.memory.append_history(big_entry)
 
     prompt = builder.build_system_prompt()
     history_section = prompt.split("# Recent History\n\n", 1)
     assert len(history_section) == 2
-    assert len(history_section[1]) < builder._MAX_HISTORY_CHARS + 200
+
+    enc = tiktoken.get_encoding("cl100k_base")
+    # Small margin for the truncation suffix appended after the token slice.
+    assert len(enc.encode(history_section[1])) <= builder._MAX_HISTORY_TOKENS + 50
 
 
 def test_no_recent_history_when_dream_has_processed_all(tmp_path) -> None: