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/timeout.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { resolveAgentTimeoutMs } from "./timeout.js";
+
+describe("resolveAgentTimeoutMs", () => {
+  it("uses a timer-safe sentinel for no-timeout overrides", () => {
+    expect(resolveAgentTimeoutMs({ overrideSeconds: 0 })).toBe(2_147_000_000);
+    expect(resolveAgentTimeoutMs({ overrideMs: 0 })).toBe(2_147_000_000);
+  });
+
+  it("clamps very large timeout overrides to timer-safe values", () => {
+    expect(resolveAgentTimeoutMs({ overrideSeconds: 9_999_999 })).toBe(2_147_000_000);
+    expect(resolveAgentTimeoutMs({ overrideMs: 9_999_999_999 })).toBe(2_147_000_000);
+  });
+});