agent2: Token count (#36496)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent2/Cargo.toml

@@ -66,6 +66,7 @@ zstd.workspace = true
 
 [dev-dependencies]
 agent = { workspace = true, "features" = ["test-support"] }
+assistant_context = { workspace = true, "features" = ["test-support"] }
 ctor.workspace = true
 client = { workspace = true, "features" = ["test-support"] }
 clock = { workspace = true, "features" = ["test-support"] }