fileapi: Generate partial reply when buildsystem generation fails

In particular, the `configureLog` reply is useful for IDEs to read
`CMakeFiles/CMakeConfigureLog.yaml` when configuration fails.

Fixes: #26621

Source/cmFileAPI.h

@@ -27,8 +27,17 @@ public:
   /** Get the list of configureLog object kind versions requested.  */
   std::vector<unsigned long> GetConfigureLogVersions();
 
+  /** Identify the situation in which WriteReplies is called.  */
+  enum class IndexFor
+  {
+    Success,
+    FailedConfigure,
+    FailedCompute,
+    FailedGenerate,
+  };
+
   /** Write fileapi replies to disk.  */
-  void WriteReplies();
+  void WriteReplies(IndexFor indexFor);
 
   /** Get the "cmake" instance with which this was constructed.  */
   cmake* GetCMakeInstance() const { return this->CMakeInstance; }
@@ -157,6 +166,9 @@ private:
       This populates the "objects" field of the reply index.  */
   std::map<Object, Json::Value> ReplyIndexObjects;
 
+  /** Identify the situation in which WriteReplies was called.  */
+  IndexFor ReplyIndexFor = IndexFor::Success;
+
   std::unique_ptr<Json::CharReader> JsonReader;
   std::unique_ptr<Json::StreamWriter> JsonWriter;