fix: context overflow compaction and subagent announce improvements (#11664) (thanks @tyler6204)

* initial commit

* feat: implement deriveSessionTotalTokens function and update usage tests

* Added deriveSessionTotalTokens function to calculate total tokens based on usage and context tokens.
* Updated usage tests to include cases for derived session total tokens.
* Refactored session usage calculations in multiple files to utilize the new function for improved accuracy.

* fix: restore overflow truncation fallback + changelog/test hardening (#11551) (thanks @tyler6204)

src/agents/openclaw-tools.subagents.sessions-spawn-resolves-main-announce-target-from.test.ts

@@ -95,6 +95,16 @@ describe("openclaw-tools: subagents", () => {
         patchParams = { key: params?.key, label: params?.label };
         return { ok: true };
       }
+      if (request.method === "chat.history") {
+        return {
+          messages: [
+            {
+              role: "assistant",
+              content: [{ type: "text", text: "done" }],
+            },
+          ],
+        };
+      }
       if (request.method === "sessions.delete") {
         return { ok: true };
       }