test(compression): resolve context_length inside mock in overflow-warning fixture

CI shard 4 caught 4 failures the local baseline diff missed (local env
pollution made them look pre-existing): _make_compressor() constructs
under a get_model_context_length mock but the lazy deferral (#32221)
pushed resolution past the with block, so the 96K window / 75% floor the
tests rely on never materialized. Resolve inside the mock.

tests/agent/test_turn_context_overflow_warning.py

@@ -34,7 +34,10 @@ def _make_compressor(**kwargs) -> ContextCompressor:
     # 96K context -> small-context floor raises threshold_percent to 0.75,
     # so threshold_tokens = 72_000. 73_000 is "over threshold".
     with patch("agent.context_compressor.get_model_context_length", return_value=96000):
-        return ContextCompressor(**defaults)
+        comp = ContextCompressor(**defaults)
+        # Resolve while the mock is active (lazy init, #32221).
+        _ = comp.context_length
+        return comp
 
 
 class TestShouldCompressInfo: