Fix agent panel scroll while editing a past message (cherry-pick #30394) (#30399)

gcp-cherry-pick-bot[bot] and Michael Sloan created

Cherry-picked Fix agent panel scroll while editing a past message
(#30394)

This is similar to the `block_mouse_down` method added in #20649 (which
has a very similar motivation), but is more comprehensive in stopping
mouse events. Since I want to cherry-pick this to the releases, keeping
this change just to the agent panel. In a follow-up will replace
existing use of `block_mouse_down` to instead use this.

Release Notes:

- N/A

Co-authored-by: Michael Sloan <michael@zed.dev>

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