bedrock: Add placeholder tool when summarising threads with tool history

[dsturnbull]

Bedrock Converse API requires toolConfig whenever messages have tool use
or result blocks. PR #33174 handled the case where tools are defined but
tool_choice is None. This change addresses another case: the tools array
is empty (e.g. thread summarisation) but messages still have tool blocks
from conversation history. A placeholder tool satisfies the requirement.

Testing:

Tested manually by:

1. Starting a conversation with Bedrock that uses tools
2. Triggering thread summarisation
3. Confirming summarisation now succeeds instead of failing with an API error

Release Notes:

• Fixed Bedrock thread summarization failing when conversation had tools

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[benbrandt]

thanks!