agent: Limit the size of patches generated from user edits (#34548)

Gradually remove details from a patch to keep it within the size limit.
This helps avoid using too much context when the user pastes large
files, generates files, or just makes many changes between agent
notifications.

Release Notes:

- N/A

Cargo.lock

@@ -779,6 +779,7 @@ dependencies = [
  "collections",
  "component",
  "derive_more 0.99.19",
+ "diffy",
  "editor",
  "feature_flags",
  "fs",