refactor: reorganize test mocks and improve test coverage

- Split mocks into separate files by responsibility
- Add comprehensive mock implementations for online algorithms
- Add edge case mocks (NaN, Inf, constant data, learning period)
- Improve test structure with fixtures and parametrization
- Enhance test coverage for edge cases and error conditions

Author: Desiment

tests/data_providers/test_idata_provider.py

@@ -0,0 +1,32 @@
+"""
+Mock data providers for testing.
+
+This module provides mock implementations of DataProvider for testing
+change-point detection algorithms.
+"""
+
+from tests.mocks.data_providers.constant import MockConstantDataProvider
+from tests.mocks.data_providers.edge import (
+    MockInfDataProvider,
+    MockMultivariateEdgeDataProvider,
+    MockNaNDataProvider,
+    MockNegativeDataProvider,
+    MockZeroDataProvider,
+)
+from tests.mocks.data_providers.empty import MockEmptyDataProvider
+from tests.mocks.data_providers.multivariate import MockMultivariateDataProvider
+from tests.mocks.data_providers.single import MockSingleObservationProvider
+from tests.mocks.data_providers.univariate import MockUnivariateDataProvider
+
+__all__ = [
+    "MockUnivariateDataProvider",
+    "MockMultivariateDataProvider",
+    "MockEmptyDataProvider",
+    "MockSingleObservationProvider",
+    "MockConstantDataProvider",
+    "MockNaNDataProvider",
+    "MockInfDataProvider",
+    "MockNegativeDataProvider",
+    "MockZeroDataProvider",
+    "MockMultivariateEdgeDataProvider",
+]

tests/data_providers/test_numpy_data_provider.py

@@ -0,0 +1,55 @@
+"""
+Mock constant data provider for testing.
+"""
+
+from collections.abc import Iterator
+
+from pysatl_cpd._typing import Number
+from pysatl_cpd.data_providers import DataProvider
+
+
+class MockConstantDataProvider(DataProvider[Number]):
+    """
+    Mock data provider that always returns the same value.
+
+    Useful for testing algorithms with constant data streams.
+
+    Parameters
+    ----------
+    value : Number
+        The constant value to return for each observation.
+    length : int
+        Number of observations to yield.
+    """
+
+    def __init__(self, value: Number, length: int) -> None:
+        self._value = value
+        self._length = length
+        self._call_count = 0
+
+    def __iter__(self) -> Iterator[Number]:
+        """Return iterator yielding constant value `length` times."""
+        self._call_count += 1
+        return iter([self._value] * self._length)
+
+    def __len__(self) -> int:
+        """Return number of observations."""
+        return self._length
+
+    def __getitem__(self, index: int) -> Number:
+        """Get observation at specific index."""
+        if 0 <= index < self._length:
+            return self._value
+        raise IndexError("Index out of range")
+
+    def get_call_count(self) -> int:
+        """Return number of times __iter__ was called."""
+        return self._call_count
+
+    def reset_call_count(self) -> None:
+        """Reset the call counter."""
+        self._call_count = 0
+
+    def __repr__(self) -> str:
+        """Return string representation."""
+        return f"MockConstantDataProvider(value={self._value}, length={self._length})"