fix: decouple mt5cli from pdmt5 high-level trading helpers (#76)

* fix: decouple mt5cli from pdmt5 high-level trading helpers

- Replace Mt5TradingClient type annotations with internal _Mt5ClientProtocol
- Lazy-import Mt5TradingClient in create_trading_client to avoid hard dependency
- Replace Mt5TradingError with Mt5OperationError in mt5cli validation paths
- Update exception handling to support future pdmt5 versions without Mt5TradingError
- Add test to enforce that mt5cli doesn't import high-level symbols at module level
- Update documentation to clarify dependency boundaries

mt5cli now relies only on low-level MT5 primitives:
- Mt5Config for configuration
- Mt5RuntimeError for runtime errors
- Raw MT5 methods (order_send, order_check, account_info, etc.)

This aligns with pdmt5's direction to remove high-level trading helpers and focus
on low-level MT5 access plus DataFrame/dict conversion.

Fixes #75 (dceoy/mt5cli#75)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PcGVFTVgyqzse3LLw38ber

* fix: address PR #76 review feedback on pdmt5 decoupling

- Replace Mt5TradingClient with Mt5DataClient in create_trading_client()
  so the function no longer depends on the high-level trading client
- Fix _RECOVERABLE_MT5_ERRORS in exceptions.py to use tuple unpacking
  form, removing the incorrect ternary assignment
- Add pragma: no cover to except ImportError branches in exceptions.py
  and sdk.py (dead code when pdmt5 is installed)
- Switch coverage exclude_lines to exclude_also so the default
  pragma: no cover pattern is preserved; also exclude bare ... stubs
  (Protocol method bodies) from coverage
- Correct inaccurate note in docs/api/public-contract.md: Mt5TradingClient
  is no longer required internally; Mt5TradingError is conditionally
  available but mt5cli raises Mt5OperationError for trading failures
- Update all mock patches from pdmt5.Mt5TradingClient to
  mt5cli.trading.Mt5DataClient to match the new module-level import

---------

Co-authored-by: Claude <noreply@anthropic.com>

tests/test_contracts.py

@@ -705,7 +705,7 @@ class TestStableSdkContract:
         """Trading session helper initializes and always shuts down."""
         mock_client = MagicMock()
         mocker.patch(
-            "mt5cli.trading.Mt5TradingClient",
+            "mt5cli.trading.Mt5DataClient",
             return_value=mock_client,
         )
 
@@ -734,7 +734,7 @@ class TestStableSdkContract:
         """Trading session helper shuts down even when the body raises."""
         mock_client = MagicMock()
         mocker.patch(
-            "mt5cli.trading.Mt5TradingClient",
+            "mt5cli.trading.Mt5DataClient",
             return_value=mock_client,
         )
 
@@ -828,6 +828,18 @@ def test_pdmt5_pass_through_names_removed_from_public_contract(name: str) -> Non
     assert name not in mt5cli.__all__, f"{name!r} should not be in mt5cli.__all__"
 
 
+def test_mt5cli_does_not_import_high_level_trading_symbols() -> None:
+    """mt5cli doesn't import Mt5TradingClient or Mt5TradingError at module level."""
+    trading_module = importlib.import_module("mt5cli.trading")
+    module_dict = vars(trading_module)
+    assert "Mt5TradingClient" not in module_dict, (
+        "mt5cli.trading should not import Mt5TradingClient at module level"
+    )
+    assert "Mt5TradingError" not in module_dict, (
+        "mt5cli.trading should not import Mt5TradingError at module level"
+    )
+
+
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