Forward cache_control from message-level placement in ChatDatabricks

[adamgurary]

Context

This fix came out of a hands-on end-to-end evaluation of building, tracing, and improving a LangGraph agent on Databricks.

Problem: ChatDatabricks drops cache_control unless it is inside a typed content block

ChatDatabricks forwards an Anthropic cache_control breakpoint only when it is nested inside a typed content block. A message-level key or an additional_kwargs placement is dropped silently, with no error, so prompt caching never activates for those placements.

Steps to reproduce

Send the same large static system block three ways through the same ChatDatabricks client and check warm-call cache_read per placement:

message-level sibling key {"role","content":<str>,"cache_control":...} : cache_read = 0       -> DROPPED
SystemMessage(additional_kwargs={"cache_control":...})                 : cache_read = 0       -> DROPPED
typed content block [{"type":"text","text":...,"cache_control":...}]   : cache_read = 8803    -> FORWARDED

Root cause

_convert_message_to_dict (v0.17.0) builds the outbound message as {"content": message.content} and copies nothing else from additional_kwargs. So cache_control survives only when it is nested inside a content block that lives in message.content.

Fix

_convert_message_to_dict now forwards a message-level cache_control from additional_kwargs onto the outbound message dict. The forwarding fires only when cache_control is present, so the no-breakpoint path is unchanged.

if (cache_control := message.additional_kwargs.get("cache_control")) is not None:
    message_dict["cache_control"] = cache_control

Two unit tests cover the present and absent cases.

This is a distinct bug from #446 (token-count inflation when caching is active). This PR addresses the breakpoint being dropped so caching never activates.

[adamgurary]

Hey @fanzeyi and @aravind-segu, this PR fixes a silent drop of message-level cache_control in _convert_message_to_dict. Two unit tests added. Would appreciate a review when you have a moment.

[adamgurary]

One thing on my side is blocked: the test workflow is waiting on a maintainer to click "Approve and run workflows" (first PR I've opened here, so the runs are sitting at action_required). Once that runs, CI should be green. The two new unit tests cover the present and absent cache_control cases and pass locally with 108 passed / 2 skipped.