feat: add OpenSearch chat store integration (#20796)

* feat: add OpenSearch chat store integration

Add a new llama-index-storage-chat-store-opensearch package that
implements BaseChatStore backed by OpenSearch. Stores chat messages
as individual documents keyed by session_id with integer indices
for ordering. Includes full sync and async support.

Closes #15926

* refactor: remove guarded imports, opensearch-py is a direct dependency

* docs: expand README with installation and usage examples

Author: eureka928

llama-index-integrations/storage/chat_store/llama-index-storage-chat-store-opensearch/Makefile

@@ -0,0 +1,17 @@
+GIT_ROOT ?= $(shell git rev-parse --show-toplevel)
+
+help:	## Show all Makefile targets.
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[33m%-30s\033[0m %s\n", $$1, $$2}'
+
+format:	## Run code autoformatters (black).
+	pre-commit install
+	git ls-files | xargs pre-commit run black --files
+
+lint:	## Run linters: pre-commit (black, ruff, codespell) and mypy
+	pre-commit install && git ls-files | xargs pre-commit run --show-diff-on-failure --files
+
+test:	## Run tests via pytest.
+	pytest tests
+
+watch-docs:	## Build and watch documentation.
+	sphinx-autobuild docs/ docs/_build/html --open-browser --watch $(GIT_ROOT)/llama_index/

llama-index-integrations/storage/chat_store/llama-index-storage-chat-store-opensearch/README.md

@@ -0,0 +1,52 @@
+# LlamaIndex Chat Store Integration: OpenSearch
+
+## Installation
+
+```
+pip install llama-index-storage-chat-store-opensearch
+```
+
+## Usage
+
+Using `OpensearchChatStore`, you can store your chat history in OpenSearch, without having to worry about manually persisting and loading the chat history.
+
+```python
+from llama_index.storage.chat_store.opensearch import OpensearchChatStore
+from llama_index.core.memory import ChatMemoryBuffer
+
+chat_store = OpensearchChatStore(
+    opensearch_url="https://localhost:9200",
+)
+
+chat_memory = ChatMemoryBuffer.from_defaults(
+    token_limit=3000,
+    chat_store=chat_store,
+    chat_store_key="user1",
+)
+```
+
+### Custom Index Name
+
+```python
+chat_store = OpensearchChatStore(
+    opensearch_url="https://localhost:9200",
+    index="my_chat_store",
+)
+```
+
+### Using a Pre-configured Client
+
+```python
+from opensearchpy import OpenSearch
+
+client = OpenSearch(
+    "https://localhost:9200",
+    http_auth=("admin", "admin"),
+    verify_certs=False,
+)
+
+chat_store = OpensearchChatStore(
+    opensearch_url="https://localhost:9200",
+    os_client=client,
+)
+```

llama-index-integrations/storage/chat_store/llama-index-storage-chat-store-opensearch/llama_index/storage/chat_store/opensearch/__init__.py

@@ -0,0 +1,3 @@
+from llama_index.storage.chat_store.opensearch.base import OpensearchChatStore
+
+__all__ = ["OpensearchChatStore"]