Add memory store kwargs to DatabricksOpenAI conversations.create

[jennsun]

Summary

Databricks agent memory attaches conversations to a Unity Catalog memory store via memory_store and scope fields on the conversations endpoint. These aren't part of the OpenAI API, so callers previously had to pass them through extra_body:

client = DatabricksOpenAI(use_ai_gateway=True)
conversation = client.conversations.create(
    extra_body={
        "memory_store": {"name": "main.default.support_agent_memory"},
        "scope": {"kind": "user", "value": user_id},
    },
)

This PR adds DatabricksConversations / AsyncDatabricksConversations resources (following the existing DatabricksResponses pattern) that accept first-class kwargs and translate them into extra_body before delegating to the stock OpenAI SDK resource:

conversation = client.conversations.create(
    store="main.default.support_agent_memory",
    scope=user_client,  # SDK extracts the user id; pass a string for external (non-Databricks) users
)

• scope accepts either a WorkspaceClient (the authenticated user's id is extracted via current_user.me().user_name) or a plain string.
• scope_kind is exposed for non-user scope kinds (defaults to "user").
• All standard conversations.create arguments (items, metadata, extra_body, extra_headers, ...) pass through unchanged, so existing extra_body usage keeps working; explicit kwargs win over the same keys in a caller-provided extra_body.
• client.conversations.items.* and the other conversation methods are inherited untouched from the OpenAI SDK.

No databricks-sdk changes required — the Databricks conversation endpoints are OpenAI-shaped, so this is purely client-side sugar over the gateway's OpenAI-compatible surface.

Test plan

• New unit tests in tests/unit_tests/test_clients.py covering: kwarg-to-extra_body translation, WorkspaceClient scope extraction, scope kind override, merging with caller-provided extra_body (kwargs take precedence), passthrough when no memory kwargs are given, backward-compatible raw extra_body usage, and the async variants.
• python -m pytest tests/unit_tests/ — 222 passed.
• ruff check / ruff format --check clean.

🤖 Generated with Claude Code