There are many places in the code that wants to work with content of
"tool use" and "tool result" messages. Currently such code uses
is_tool_use_message() and is_tool_result() message to check if message
is of right kind, and then pokes into message internals. This is not
very efficient.
Introduce get_tool_use_data() and get_tool_result_data() that would
return contents of the message if it is of right kind, or nil otherwise.
Also introduce get_tool_result() that attempts to locate result of a
tool execution by its invocation ID.
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