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>

README.md

@@ -246,7 +246,7 @@ update_history_with_config(
 - **Rate view resolution**: use `resolve_rate_view_name()` / `resolve_rate_view_names()` to map symbols and granularities to existing SQLite compatibility views without creating databases. Both accept `None` (or a missing path) and return deterministic default names unless `require_existing=True`.
 - **Rate view loading**: use `load_rate_data()` / `load_rate_data_from_connection()` to load a SQLite rate table or view into a `DatetimeIndex` DataFrame.
 - **Multi-series rate loading**: use `build_rate_targets()` to build neutral `RateTarget(symbol, timeframe)` pairs, `resolve_rate_tables()` to map them to table/view names (pass `require_existing=True` for strict resolution), and `load_rate_series_from_sqlite()` to load them into a mapping keyed by `(symbol, integer timeframe)`. The loader requires existing managed views unless `explicit_tables` is supplied, and rejects duplicate `(symbol, timeframe)` targets.
-- **Multi-account latest rates**: use `collect_latest_rates_for_accounts()` with `AccountSpec` to read the latest bars for several account groups, merged into a `(symbol, integer timeframe)` mapping. For long-running pollers, `collect_latest_rates_for_accounts_with_retries()` adds bounded exponential backoff that retries only `pdmt5.Mt5TradingError` / `pdmt5.Mt5RuntimeError` and re-raises once `retry_count` is exhausted.
+- **Multi-account latest rates**: use `collect_latest_rates_for_accounts()` with `AccountSpec` to read the latest bars for several account groups, merged into a `(symbol, integer timeframe)` mapping. For long-running pollers, `collect_latest_rates_for_accounts_with_retries()` adds bounded exponential backoff that retries only recoverable MT5 errors and re-raises once `retry_count` is exhausted.
 - **Latest closed bars**: use `collect_latest_closed_rates_for_accounts()` when downstream logic must exclude the still-forming current bar. It fetches `count + 1` bars at `start_pos=0`, drops the last row with `drop_forming_rate_bar()`, and validates each series is non-empty. `collect_latest_closed_rates_by_granularity()` returns the same data keyed by `(symbol, granularity_name)` such as `("EURUSD", "M1")`.
 
 ```python
@@ -263,7 +263,7 @@ eurusd_m1 = rates["EURUSD", "M1"]  # closed bars only
 
 - **Credential resolution**: use `resolve_account_spec()` / `resolve_account_specs()` to merge explicit override values over `AccountSpec` fields and expand `${ENV_VAR}` placeholders (via `substitute_env_placeholders()`), raising `ValueError` for missing variables. This keeps secrets out of plan/config files without coupling to any strategy code. For config dicts or nested structures loaded from YAML/TOML, use `substitute_mapping_values(data, keys={"login", "password"})` to expand placeholders only for caller-specified keys — key names are never hard-coded in mt5cli.
 - **Throttled history updates**: use `ThrottledHistoryUpdater` to wrap `update_history()` with a minimum `interval_seconds` between successful runs (monotonic clock). Call `should_update()` / `update(client, symbols)` from an application loop; errors propagate by default, or pass `suppress_errors=True` to swallow recoverable `Mt5*Error`, `sqlite3.Error`, `ValueError`, `OSError`, and MT5 client capability errors for history API methods without advancing the throttle (other `AttributeError` / `TypeError` values always propagate). Pass `update_backend` to inject a custom history update callable (same keyword arguments as `update_history`) instead of monkey-patching `mt5cli.sdk.update_history`.
-- **Trading session helpers**: use `mt5_trading_session()` for a trading-capable `pdmt5.Mt5TradingClient` that initializes/logs in via `Mt5Config.path` and always shuts down safely. Pair with `detect_position_side()`, `calculate_margin_and_volume()`, and `determine_order_limits()` for generic position and sizing utilities. Keep read-only collection on `mt5_session()` / `MT5Client`.
+- **Trading session helpers**: use `mt5_trading_session()` for a trading-capable client that initializes/logs in via `Mt5Config.path` and always shuts down safely. Pair with `detect_position_side()`, `calculate_margin_and_volume()`, and `determine_order_limits()` for generic position and sizing utilities. Keep read-only collection on `mt5_session()` / `MT5Client`.
 - **Granularity-keyed rate loading**: `load_rate_series_by_granularity()` builds targets with `build_rate_targets()`, loads them with `load_rate_series_from_sqlite()`, and returns a mapping keyed by `(symbol | None, granularity_name)` such as `("EURUSD", "M1")` to reduce downstream boilerplate.
 - **MT5 session helper**: use the `mt5_session()` context manager to attach to (or, when `Mt5Config.path` is set, launch) an MT5 terminal, log in, and yield a connected `MT5Client` that shuts down on exit.
 - **SQLite export helpers**: use `export_dataframe_to_sqlite()` for append mode, optional index export, and post-write deduplication by key columns.