refactor(history): add helpers to get content of tool "use" and "result"

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.