feat: add PageIndex SDK with local/cloud dual-mode support (#207)

pageindex/errors.py

@@ -0,0 +1,28 @@
+class PageIndexError(Exception):
+    """Base exception for all PageIndex SDK errors."""
+    pass
+
+
+class CollectionNotFoundError(PageIndexError):
+    """Collection does not exist."""
+    pass
+
+
+class DocumentNotFoundError(PageIndexError):
+    """Document ID not found."""
+    pass
+
+
+class IndexingError(PageIndexError):
+    """Indexing pipeline failure."""
+    pass
+
+
+class CloudAPIError(PageIndexError):
+    """Cloud API returned error."""
+    pass
+
+
+class FileTypeError(PageIndexError):
+    """Unsupported file type."""
+    pass