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

tests/test_local_backend.py

@@ -141,3 +141,17 @@ def test_normalize_doc_ids():
 def test_normalize_doc_ids_rejects_empty_list():
     with pytest.raises(ValueError, match="cannot be empty"):
         LocalBackend._normalize_doc_ids([])
+
+
+# ── error taxonomy: missing docs raise DocumentNotFoundError ─────────────────
+
+def test_get_document_missing_raises(backend):
+    backend.get_or_create_collection("papers")
+    with pytest.raises(DocumentNotFoundError, match="ghost"):
+        backend.get_document("papers", "ghost")
+
+
+def test_delete_document_missing_raises(backend):
+    backend.get_or_create_collection("papers")
+    with pytest.raises(DocumentNotFoundError, match="ghost"):
+        backend.delete_document("papers", "ghost")