feat: compaction prompt configurable (#5959)

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

codex-rs/mcp-server/src/codex_tool_config.rs

@@ -49,6 +49,10 @@ pub struct CodexToolCallParam {
     /// The set of instructions to use instead of the default ones.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub base_instructions: Option<String>,
+
+    /// Prompt used when compacting the conversation.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub compact_prompt: Option<String>,
 }
 
 /// Custom enum mirroring [`AskForApproval`], but has an extra dependency on
@@ -141,6 +145,7 @@ impl CodexToolCallParam {
             sandbox,
             config: cli_overrides,
             base_instructions,
+            compact_prompt,
         } = self;
 
         // Build the `ConfigOverrides` recognized by codex-core.
@@ -154,6 +159,7 @@ impl CodexToolCallParam {
             model_provider: None,
             codex_linux_sandbox_exe,
             base_instructions,
+            compact_prompt,
             include_apply_patch_tool: None,
             include_view_image_tool: None,
             show_raw_agent_reasoning: None,
@@ -288,6 +294,10 @@ mod tests {
                 "description": "The set of instructions to use instead of the default ones.",
                 "type": "string"
               },
+              "compact-prompt": {
+                "description": "Prompt used when compacting the conversation.",
+                "type": "string"
+              },
             },
             "required": [
               "prompt"