fix(signal_v2-phase3b): use BaseChronosPipeline for new model architectures

ChronosPipeline (legacy T5) does not support amazon/chronos-2 or
chronos-bolt-* (input_patch_size). Switch to BaseChronosPipeline
which auto-detects variant and returns the appropriate sub-pipeline
(ChronosBoltPipeline / Chronos2Pipeline / ChronosPipeline).

Also handle the dtype kwarg deprecation: try newer `dtype=` first,
fall back to `torch_dtype=` for older versions.

Test mock_pipeline fixture updated to patch BaseChronosPipeline.

45/45 tests pass. Verified amazon/chronos-bolt-base loads on CUDA.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

signal_v2/tests/test_chronos_predictor.py

@@ -7,8 +7,9 @@ import pytest
 
 @pytest.fixture
 def mock_pipeline():
-    """Mock ChronosPipeline.from_pretrained returning a mock pipeline object."""
-    with patch("chronos.ChronosPipeline") as cls:
+    """Mock BaseChronosPipeline.from_pretrained returning a mock pipeline object."""
+    with patch("chronos.BaseChronosPipeline") as cls:
+        cls.__name__ = "BaseChronosPipeline"
         instance = MagicMock()
         cls.from_pretrained.return_value = instance
         yield instance