agent: Push diffs of user edits to the agent (#34487)

This change improves user/agent collaborative editing.

When the user edits files that are used by the agent, the
`project_notification` tool now pushes *diffs* of the changes, not just
file names. This helps the agent to stay up to date without needing to
re-read files.

Release Notes:

- Improved user/agent collaborative editing: agent now receives diffs of
user edits

crates/assistant_tool/Cargo.toml

@@ -40,6 +40,7 @@ collections = { workspace = true, features = ["test-support"] }
 clock = { workspace = true, features = ["test-support"] }
 ctor.workspace = true
 gpui = { workspace = true, features = ["test-support"] }
+indoc.workspace = true
 language = { workspace = true, features = ["test-support"] }
 language_model = { workspace = true, features = ["test-support"] }
 log.workspace = true