feat:compatible with Pageindex SDK (#238)

* feat:compatible with Pageindex SDK

* corner cases fixed

* fix: mock behavior of old SDK

* fix: close streaming response and warn on empty api_key

- LegacyCloudAPI: close response in `finally` for both _stream_chat_response
  variants so abandoned iterators no longer leak the TCP connection.
- PageIndexClient: emit a warning instead of silently falling back to local
  when api_key is the empty string, surfacing typical env-var-unset misconfig.
- FakeResponse: add close()/closed to match the real requests.Response API.
- Add unit coverage for stream close (both paths) and the empty-api_key warning.
- Add scripts/e2e_legacy_sdk.py to smoke-test the legacy SDK contract end-to-end
  against api.pageindex.ai.

* chore: mark legacy SDK methods with @deprecated and docstring pointers

- Decorate the 12 PageIndexClient cloud-SDK compat methods with
  @typing_extensions.deprecated(..., category=PendingDeprecationWarning):
  - IDE/type-checkers render them with a strikethrough hint
  - runtime warnings stay silent by default (no spam for existing callers),
    surfaceable via `python -W default::PendingDeprecationWarning`
- Add a one-line docstring on each pointing to the Collection-based equivalent.
- Promote typing-extensions to a direct dependency (was transitive via litellm).

---------

Co-authored-by: XinyanZhou <xinyanzhou@XinyanZhoudeMacBook-Pro.local>
Co-authored-by: saccharin98 <xinyanzhou938@gmail.com>
Co-authored-by: mountain <kose2livs@gmail.com>

pageindex/__init__.py

@@ -13,6 +13,7 @@ from .storage.protocol import StorageEngine
 from .events import QueryEvent
 from .errors import (
     PageIndexError,
+    PageIndexAPIError,
     CollectionNotFoundError,
     DocumentNotFoundError,
     IndexingError,
@@ -32,6 +33,7 @@ __all__ = [
     "StorageEngine",
     "QueryEvent",
     "PageIndexError",
+    "PageIndexAPIError",
     "CollectionNotFoundError",
     "DocumentNotFoundError",
     "IndexingError",