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/errors.py

@@ -27,8 +27,16 @@ class PageIndexAPIError(PageIndexError):
 
 
 class CloudAPIError(PageIndexAPIError):
-    """Cloud API returned error."""
-    pass
+    """Cloud API returned error.
+
+    ``status_code`` carries the HTTP status when the error came from an HTTP
+    response (None for transport-level failures), so callers can branch on it
+    instead of parsing the message.
+    """
+
+    def __init__(self, message: str, status_code: int | None = None):
+        super().__init__(message)
+        self.status_code = status_code
 
 
 class FileTypeError(PageIndexError):