Support openai/form extended form elicitations (#27500)

# Summary
Allow App Server clients to opt into `openai/form` MCP elicitations.

codex-rs/app-server-protocol/src/protocol/v2/mcp.rs

@@ -632,6 +632,15 @@ pub enum McpServerElicitationRequest {
         message: String,
         requested_schema: McpElicitationSchema,
     },
+    #[serde(rename = "openai/form", rename_all = "camelCase")]
+    #[ts(rename = "openai/form", rename_all = "camelCase")]
+    OpenAiForm {
+        #[serde(rename = "_meta")]
+        #[ts(rename = "_meta")]
+        meta: Option<JsonValue>,
+        message: String,
+        requested_schema: JsonValue,
+    },
     #[serde(rename_all = "camelCase")]
     #[ts(rename_all = "camelCase")]
     Url {
@@ -658,6 +667,15 @@ impl TryFrom<CoreElicitationRequest> for McpServerElicitationRequest {
                 message,
                 requested_schema: serde_json::from_value(requested_schema)?,
             }),
+            CoreElicitationRequest::OpenAiForm {
+                meta,
+                message,
+                requested_schema,
+            } => Ok(Self::OpenAiForm {
+                meta,
+                message,
+                requested_schema,
+            }),
             CoreElicitationRequest::Url {
                 meta,
                 message,

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -1901,6 +1901,40 @@ fn mcp_server_elicitation_request_from_core_form_request() {
     );
 }
 
+#[test]
+fn mcp_server_elicitation_request_from_core_openai_form_request() {
+    let requested_schema = json!({
+        "type": "object",
+        "properties": {
+            "template": {
+                "type": "openai/imagePicker",
+                "title": "Template",
+                "items": [{
+                    "id": "monthly-review",
+                    "title": "Monthly review",
+                    "image": "data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciLz4=",
+                }],
+            },
+        },
+        "required": ["template"],
+    });
+    let request = McpServerElicitationRequest::try_from(CoreElicitationRequest::OpenAiForm {
+        meta: None,
+        message: "Choose a report".to_string(),
+        requested_schema: requested_schema.clone(),
+    })
+    .expect("OpenAI form request should convert");
+
+    assert_eq!(
+        request,
+        McpServerElicitationRequest::OpenAiForm {
+            meta: None,
+            message: "Choose a report".to_string(),
+            requested_schema,
+        }
+    );
+}
+
 #[test]
 fn mcp_elicitation_schema_matches_mcp_2025_11_25_primitives() {
     let schema: McpElicitationSchema = serde_json::from_value(json!({