feat(sqlserver): add SQLServerChatMessageHistory

[DevNinja (0xDevNinja)]

Summary

Adds SQLServerChatMessageHistory, an implementation of langchain_core.chat_history.BaseChatMessageHistory backed by a SQL Server / Azure SQL table. This is the chat_message_histories integration tracked in #14.

Messages are JSON-serialized via message_to_dict and stored in an NVARCHAR(MAX) column, keyed by session_id, with an auto-incrementing id column that preserves insertion order on read. The class supports the same connection-string conventions as SQLServer_VectorStore — including Entra ID authentication when the connection string carries no credentials — so users can reuse existing config.

Closes #14

Public API

from langchain_sqlserver import SQLServerChatMessageHistory

history = SQLServerChatMessageHistory(
    session_id="user-123",
    connection_string="Driver={ODBC Driver 18 for SQL Server};Server=tcp:host,1433;Database=mydb;Uid=user;Pwd=pwd;TrustServerCertificate=yes;",
)
history.add_user_message("hi")
history.add_ai_message("hello")
print(history.messages)

Works out of the box with RunnableWithMessageHistory.

Files

• langchain_sqlserver/chat_message_histories.py — new module with SQLServerChatMessageHistory.
• langchain_sqlserver/__init__.py — export the new class.
• tests/unit_tests/test_chat_message_histories.py — 6 unit tests covering validation, Entra-ID-vs-uid/pwd routing, serialization and the empty-input fast path. Does not require a live DB.
• tests/unit_tests/test_imports.py — extend the __all__ snapshot.
• tests/integration_tests/test_chat_message_histories.py — 6 integration tests (round-trip persistence, helper methods, session isolation, scoped clear, empty-batch noop, empty-session_id rejection). Uses the existing TEST_AZURESQLSERVER_TRUSTED_CONNECTION / TEST_PYODBC_CONNECTION_STRING env vars.

Test plan

• ruff check langchain_sqlserver tests — clean
• mypy langchain_sqlserver tests — clean (14 source files)
• pytest tests/unit_tests/ — 7 passed (6 new + import snapshot), 1 module skipped (pre-existing)
• pytest tests/integration_tests/test_chat_message_histories.py against a live SQL Server / Azure SQL instance.

Notes

• The connection-string parsing and Entra-ID token-provider helpers intentionally mirror those on SQLServer_VectorStore so users get identical auth semantics across both classes. Extracting the helpers into a shared internal module is left as a follow-up so this PR stays additive.