feat(mcp): sérialisation Polars -> JSON pour les tools MCP

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/mcp/test_serialization.py

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+import datetime
+
+import polars as pl
+
+from src.mcp.serialization import to_json_records
+
+
+def test_dates_become_iso_strings():
+    df = pl.DataFrame({"d": [datetime.date(2025, 1, 1)], "n": [10]})
+    assert to_json_records(df) == [{"d": "2025-01-01", "n": 10}]
+
+
+def test_none_is_preserved():
+    df = pl.DataFrame({"nom": [None], "x": [3]})
+    assert to_json_records(df) == [{"nom": None, "x": 3}]
+
+
+def test_strings_and_numbers_untouched():
+    df = pl.DataFrame({"s": ["abc"], "f": [1.5]})
+    assert to_json_records(df) == [{"s": "abc", "f": 1.5}]