fix(config): validate final_tokens > 0 + add replace() revalidates regression test

tests/test_config.py

@@ -58,3 +58,19 @@ def test_effective_final_tokens_falls_back_to_chunk():
     assert cfg.effective_final_tokens() == 8192
     cfg2 = RSAConfig(chunk_tokens=8192, final_tokens=20000)
     assert cfg2.effective_final_tokens() == 20000
+
+
+def test_replace_revalidates():
+    cfg = RSAConfig()
+    with pytest.raises(ValueError, match="aggregation_subsample.*<= parallel"):
+        cfg.replace(parallel=2, aggregation_subsample=10)
+
+
+def test_final_tokens_zero_rejected():
+    with pytest.raises(ValueError, match="final_tokens must be positive"):
+        RSAConfig(final_tokens=0)
+
+
+def test_final_tokens_negative_rejected():
+    with pytest.raises(ValueError, match="final_tokens must be positive"):
+        RSAConfig(final_tokens=-1)