agent2: Port read_file tool (#35840)

Ports the read_file tool from `assistant_tools` to `agent2`. 

Note: Image support not implemented.

Release Notes:

- N/A

crates/agent2/Cargo.toml

@@ -24,6 +24,8 @@ futures.workspace = true
 gpui.workspace = true
 handlebars = { workspace = true, features = ["rust-embed"] }
 indoc.workspace = true
+itertools.workspace = true
+language.workspace = true
 language_model.workspace = true
 language_models.workspace = true
 log.workspace = true
@@ -55,3 +57,4 @@ project = { workspace = true, "features" = ["test-support"] }
 reqwest_client.workspace = true
 settings = { workspace = true, "features" = ["test-support"] }
 worktree = { workspace = true, "features" = ["test-support"] }
+pretty_assertions.workspace = true