split TerminationPending into two values, keeping ComputeStatus stateless (#12506)

After https://github.com/neondatabase/neon/pull/12240 we observed
issues in our go code as `ComputeStatus` is not stateless, thus doesn't
deserialize as string.

```
could not check compute activity: json: cannot unmarshal object into Go struct field
ComputeState.status of type computeclient.ComputeStatus
```

- Fix this by splitting this status into two.
- Update compute OpenApi spec to reflect changes to `/terminate` in
previous PR

compute_tools/src/compute.rs

@@ -956,14 +956,20 @@ impl ComputeNode {
             None
         };
 
-        let mut delay_exit = false;
         let mut state = self.state.lock().unwrap();
         state.terminate_flush_lsn = lsn;
-        if let ComputeStatus::TerminationPending { mode } = state.status {
+
+        let delay_exit = state.status == ComputeStatus::TerminationPendingFast;
+        if state.status == ComputeStatus::TerminationPendingFast
+            || state.status == ComputeStatus::TerminationPendingImmediate
+        {
+            info!(
+                "Changing compute status from {} to {}",
+                state.status,
+                ComputeStatus::Terminated
+            );
             state.status = ComputeStatus::Terminated;
             self.state_changed.notify_all();
-            // we were asked to terminate gracefully, don't exit to avoid restart
-            delay_exit = mode == compute_api::responses::TerminateMode::Fast
         }
         drop(state);
 
@@ -1901,7 +1907,8 @@ impl ComputeNode {
 
                             // exit loop
                             ComputeStatus::Failed
-                            | ComputeStatus::TerminationPending { .. }
+                            | ComputeStatus::TerminationPendingFast
+                            | ComputeStatus::TerminationPendingImmediate
                             | ComputeStatus::Terminated => break 'cert_update,
 
                             // wait

compute_tools/src/http/openapi_spec.yaml

@@ -371,9 +371,28 @@ paths:
       summary: Terminate Postgres and wait for it to exit
       description: ""
       operationId: terminate
+      parameters:
+        - name: mode
+          in: query
+          description: "Terminate mode: fast (wait 30s before returning) and immediate"
+          required: false
+          schema:
+            type: string
+            enum: ["fast", "immediate"]
+            default: fast
       responses:
         200:
           description: Result
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/TerminateResponse"
+        201:
+          description: Result if compute is already terminated
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/TerminateResponse"
         412:
           description: "wrong state"
           content:
@@ -530,11 +549,14 @@ components:
       type: string
       enum:
         - empty
-        - init
-        - failed
-        - running
         - configuration_pending
+        - init
+        - running
         - configuration
+        - failed
+        - termination_pending_fast
+        - termination_pending_immediate
+        - terminated
       example: running
 
     ExtensionInstallRequest:
@@ -660,6 +682,17 @@ components:
           description: Role name.
           example: "neon"
 
+    TerminateResponse:
+      type: object
+      required:
+        - lsn
+      properties:
+        lsn:
+          type: string
+          nullable: true
+          description: "last WAL flush LSN"
+          example: "0/028F10D8"
+
     SetRoleGrantsResponse:
       type: object
       required:

compute_tools/src/http/routes/terminate.rs

@@ -3,7 +3,7 @@ use crate::http::JsonResponse;
 use axum::extract::State;
 use axum::response::Response;
 use axum_extra::extract::OptionalQuery;
-use compute_api::responses::{ComputeStatus, TerminateResponse};
+use compute_api::responses::{ComputeStatus, TerminateMode, TerminateResponse};
 use http::StatusCode;
 use serde::Deserialize;
 use std::sync::Arc;
@@ -12,7 +12,7 @@ use tracing::info;
 
 #[derive(Deserialize, Default)]
 pub struct TerminateQuery {
-    mode: compute_api::responses::TerminateMode,
+    mode: TerminateMode,
 }
 
 /// Terminate the compute.
@@ -24,16 +24,16 @@ pub(in crate::http) async fn terminate(
     {
         let mut state = compute.state.lock().unwrap();
         if state.status == ComputeStatus::Terminated {
-            return JsonResponse::success(StatusCode::CREATED, state.terminate_flush_lsn);
+            let response = TerminateResponse {
+                lsn: state.terminate_flush_lsn,
+            };
+            return JsonResponse::success(StatusCode::CREATED, response);
         }
 
         if !matches!(state.status, ComputeStatus::Empty | ComputeStatus::Running) {
             return JsonResponse::invalid_status(state.status);
         }
-        state.set_status(
-            ComputeStatus::TerminationPending { mode },
-            &compute.state_changed,
-        );
+        state.set_status(mode.into(), &compute.state_changed);
     }
 
     forward_termination_signal(false);

compute_tools/src/monitor.rs

@@ -84,7 +84,8 @@ impl ComputeMonitor {
         if matches!(
             compute_status,
             ComputeStatus::Terminated
-                | ComputeStatus::TerminationPending { .. }
+                | ComputeStatus::TerminationPendingFast
+                | ComputeStatus::TerminationPendingImmediate
                 | ComputeStatus::Failed
         ) {
             info!(