acp: Update agent-client-protocol SDK to 2.0.0 (#61570)

This isn't ACP v2, but just a new version of the SDK where we had to do
some breaking changes + cleanups.

Release Notes:

- N/A

Cargo.toml

@@ -514,7 +514,7 @@ accesskit = { version = "0.24.0", features = ["enumn"] }
 accesskit_macos = "0.26.0"
 accesskit_unix = "0.21.0"
 accesskit_windows = "0.33.1"
-agent-client-protocol = { version = "=1.3.0", features = ["unstable"] }
+agent-client-protocol = { version = "=2.0.0", features = ["unstable"] }
 aho-corasick = "1.1"
 alacritty_terminal = { git = "https://github.com/zed-industries/alacritty", rev = "4c129667ce56611becdc82de6e28218c80e2e88f" }
 any_vec = "0.14"