assistant edit tool: Track read buffers and notify model of user edits (#26952)

When the model reads file, we'll track the version it read, and let it
know if the user makes edits to the buffer. This helps prevent edit
failures because it'll know to re-read the file before.

Release Notes:

- N/A

crates/assistant_tool/Cargo.toml

@@ -14,6 +14,7 @@ path = "src/assistant_tool.rs"
 [dependencies]
 anyhow.workspace = true
 collections.workspace = true
+clock.workspace = true
 derive_more.workspace = true
 gpui.workspace = true
 language.workspace = true