fix(cloud): align cloud/local backend contracts and harden the HTTP layer

Fixes the cloud/local contract mismatches from the PR #272 review
(verified against the official API docs — the cloud API has no
folder/collection endpoints publicly, GET /docs supports limit<=100
with offset):

- query_stream: emit a terminal answer_done event with the full answer
  (same contract as the local backend); raise CloudAPIError instead of
  disguising HTTP errors as answer events; move the initial connect
  inside try so a connection failure can no longer strand the consumer
  awaiting a sentinel that never arrives
- _request: rewind file objects before retrying so a transient 5xx/429
  during upload no longer re-sends an empty multipart body; carry the
  HTTP status on CloudAPIError (status_code) and keep the last status
  in the max-retries error
- list_documents: paginate with limit/offset instead of a hard-coded
  limit=100, so >100-doc collections are no longer silently truncated
  (whole-collection queries rely on this list)
- folders: treat only 403/404 as "folders unavailable" (warned via
  warnings.warn instead of an invisible logger.warning, matched on
  status_code instead of a "403" substring); transient errors now
  propagate instead of being permanently cached as folder_id=None
- error taxonomy parity: cloud doc endpoints map HTTP 404 to
  DocumentNotFoundError; local get_document raises DocumentNotFoundError
  instead of returning {}; local delete_document raises on missing
  doc_id instead of silently deleting nothing
- cloud get_document warns that include_text is not supported instead
  of silently ignoring it

Adds regression tests for each fix (11 new tests).

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

pageindex/backend/local.py

@@ -143,7 +143,7 @@ class LocalBackend:
         """
         doc = self._storage.get_document(collection, doc_id)
         if not doc:
-            return {}
+            raise DocumentNotFoundError(f"Document {doc_id} not found")
         doc["structure"] = self._storage.get_document_structure(collection, doc_id)
         if include_text:
             pages = self._storage.get_pages(collection, doc_id) or []
@@ -190,7 +190,11 @@ class LocalBackend:
 
     def delete_document(self, collection: str, doc_id: str) -> None:
         doc = self._storage.get_document(collection, doc_id)
-        if doc and doc.get("file_path"):
+        if not doc:
+            # Parity with the cloud backend, which surfaces HTTP 404 as
+            # DocumentNotFoundError — a typo'd doc_id should not pass silently.
+            raise DocumentNotFoundError(f"Document {doc_id} not found")
+        if doc.get("file_path"):
             Path(doc["file_path"]).unlink(missing_ok=True)
         # Clean up images directory: files/{collection}/{doc_id}/
         doc_dir = self._files_dir / collection / doc_id