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/usage.ts

@@ -103,3 +103,34 @@ export function derivePromptTokens(usage?: {
   const sum = input + cacheRead + cacheWrite;
   return sum > 0 ? sum : undefined;
 }
+
+export function deriveSessionTotalTokens(params: {
+  usage?: {
+    input?: number;
+    total?: number;
+    cacheRead?: number;
+    cacheWrite?: number;
+  };
+  contextTokens?: number;
+}): number | undefined {
+  const usage = params.usage;
+  if (!usage) {
+    return undefined;
+  }
+  const input = usage.input ?? 0;
+  const promptTokens = derivePromptTokens({
+    input: usage.input,
+    cacheRead: usage.cacheRead,
+    cacheWrite: usage.cacheWrite,
+  });
+  let total = promptTokens ?? usage.total ?? input;
+  if (!(total > 0)) {
+    return undefined;
+  }
+
+  const contextTokens = params.contextTokens;
+  if (typeof contextTokens === "number" && Number.isFinite(contextTokens) && contextTokens > 0) {
+    total = Math.min(total, contextTokens);
+  }
+  return total;
+}