feat: add lazy loading for optional pandas and polars modules

Introduce two new functions, _optional_pandas_module and _optional_polars_module, to lazily import and cache the pandas and polars libraries. This change reduces overhead in hot paths by avoiding unnecessary imports when these libraries are not available. Update existing wrappers to utilize these functions for improved performance and cleaner error handling.

tests/unit/test_optional_dependency_wrappers.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import numpy as np
+
+from ferro_ta._utils import (
+    _optional_pandas_module,
+    _optional_polars_module,
+    pandas_wrap,
+    polars_wrap,
+)
+
+
+def _missing_only(module_name: str):
+    real_import = __import__
+    attempts: list[str] = []
+
+    def side_effect(name, globals=None, locals=None, fromlist=(), level=0):
+        if name == module_name:
+            attempts.append(name)
+            raise ImportError(f"{module_name} not installed")
+        return real_import(name, globals, locals, fromlist, level)
+
+    return attempts, side_effect
+
+
+def test_pandas_wrap_caches_missing_optional_import() -> None:
+    _optional_pandas_module.cache_clear()
+    wrapped = pandas_wrap(lambda arr: arr)
+    arr = np.array([1.0, 2.0, 3.0], dtype=np.float64)
+    attempts, side_effect = _missing_only("pandas")
+
+    try:
+        with patch("builtins.__import__", side_effect=side_effect):
+            np.testing.assert_array_equal(wrapped(arr), arr)
+            np.testing.assert_array_equal(wrapped(arr), arr)
+    finally:
+        _optional_pandas_module.cache_clear()
+
+    assert attempts == ["pandas"]
+
+
+def test_polars_wrap_caches_missing_optional_import() -> None:
+    _optional_polars_module.cache_clear()
+    wrapped = polars_wrap(lambda arr: arr)
+    arr = np.array([1.0, 2.0, 3.0], dtype=np.float64)
+    attempts, side_effect = _missing_only("polars")
+
+    try:
+        with patch("builtins.__import__", side_effect=side_effect):
+            np.testing.assert_array_equal(wrapped(arr), arr)
+            np.testing.assert_array_equal(wrapped(arr), arr)
+    finally:
+        _optional_polars_module.cache_clear()
+
+    assert attempts == ["polars"]