feat: compaction prompt configurable (#5959)

```
 codex -c compact_prompt="Summarize in bullet points"
 ```

codex-rs/app-server-protocol/src/protocol.rs

@@ -321,6 +321,10 @@ pub struct NewConversationParams {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub base_instructions: Option<String>,
 
+    /// Prompt used during conversation compaction.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub compact_prompt: Option<String>,
+
     /// Whether to include the apply patch tool in the conversation.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub include_apply_patch_tool: Option<bool>,
@@ -1125,6 +1129,7 @@ mod tests {
                 sandbox: None,
                 config: None,
                 base_instructions: None,
+                compact_prompt: None,
                 include_apply_patch_tool: None,
             },
         };