acp: Support session modes (e.g. CC plan mode) (#37632)

Adds support for [ACP session
modes](https://github.com/zed-industries/agent-client-protocol/pull/67)
enabling plan and other permission modes in CC:


https://github.com/user-attachments/assets/dea18d82-4da6-465e-983b-02b77c6dcf15


Release Notes:

- Claude Code: Add support for plan mode, and all other permission modes

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent_ui/src/agent_diff.rs

@@ -1529,7 +1529,8 @@ impl AgentDiff {
             | AcpThreadEvent::ToolAuthorizationRequired
             | AcpThreadEvent::PromptCapabilitiesUpdated
             | AcpThreadEvent::AvailableCommandsUpdated(_)
-            | AcpThreadEvent::Retry(_) => {}
+            | AcpThreadEvent::Retry(_)
+            | AcpThreadEvent::ModeUpdated(_) => {}
         }
     }