feat: stable MT5Client public API and infrastructure layer (#30)

* feat: add stable MT5Client public API and infrastructure layer

Introduce a reusable public API for downstream trading applications:

- MT5Client as the primary client abstraction with order_check/order_send
- schemas module with DataKind contracts, validation, and normalization
- converters, exceptions, retry, and storage facade modules
- CLI order commands now route through MT5Client
- connected_client made public; retry logic centralized
- Contract tests for API surface, schemas, and storage round-trips
- README and docs updated with Python API usage examples

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

* fix: correct time coercion, broker-safe symbols, and execution docs

- Normalize MT5 time columns with correct second/millisecond units
- Coerce all present known MT5 time fields, including optional order times
- Preserve broker symbol casing in normalize_symbol()
- Document order_send() as a live execution primitive with clear scope boundaries
- Add contract tests for timestamp and symbol normalization behavior

Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Daichi Narushima <dceoy@users.noreply.github.com>

tests/test_sdk.py

@@ -175,7 +175,7 @@ class TestConnectionLifecycle:
         mock_client = MagicMock()
         mocker.patch("mt5cli.sdk.Mt5DataClient", return_value=mock_client)
         config = MagicMock()
-        with sdk._connected_client(config):  # type: ignore[reportPrivateUsage]
+        with sdk.connected_client(config):  # type: ignore[reportPrivateUsage]
             mock_client.initialize_and_login_mt5.assert_called_once()
         mock_client.shutdown.assert_called_once()
 
@@ -191,7 +191,7 @@ class TestConnectionLifecycle:
         mocker.patch("mt5cli.sdk.Mt5DataClient", return_value=mock_client)
         with (
             pytest.raises(RuntimeError, match="login failed"),
-            sdk._connected_client(MagicMock()),  # type: ignore[reportPrivateUsage]
+            sdk.connected_client(MagicMock()),  # type: ignore[reportPrivateUsage]
         ):
             pass
         mock_client.shutdown.assert_called_once()
@@ -1342,7 +1342,7 @@ class TestCollectLatestRatesForAccounts:
         """Test account fields override base_config, empty login falls back."""
         configs: list[object] = []
 
-        def _record_config(*, config: object) -> MagicMock:
+        def _record_config(*, config: object, **_: object) -> MagicMock:
             configs.append(config)
             return mock_client