core[major]: On Tool End Observation Casting Fix (#18798)
This PR updates the on_tool_end handlers to return the raw output from the tool instead of casting it to a string.
This is technically a breaking change, though it's impact is expected to be somewhat minimal. It will fix behavior in `astream_events` as well.
Fixes the following issue #18760 raised by @eyurtsev
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Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
