fix: guard estimateMessageCharsCached against null/undefined messages

After upgrading to v2026.3.7, orphaned null/undefined entries can appear
in the session message array (likely from the new compaction lifecycle).
These cause a TypeError in estimateMessageCharsCached when WeakMap.get()
is called on a non-object, crashing active sessions.

Changes:
- Add null/typeof guard at the top of estimateMessageCharsCached()
- Add .filter() in estimateContextChars() to skip null entries before reduce
- Add test coverage for null, undefined, and primitive inputs

Regression since v2026.3.7. Related to #35146 and #35143 (null guards
for thinking/text blocks by @Sid-Qin), but this addresses null entries
in the message array itself rather than within message content blocks.

src/agents/pi-embedded-runner/tool-result-char-estimator.test.ts

@@ -0,0 +1,92 @@
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import { describe, expect, it } from "vitest";
+import { castAgentMessage } from "../test-helpers/agent-message-fixtures.js";
+import {
+  createMessageCharEstimateCache,
+  estimateContextChars,
+  estimateMessageCharsCached,
+} from "./tool-result-char-estimator.js";
+
+function makeUser(text: string): AgentMessage {
+  return castAgentMessage({
+    role: "user",
+    content: text,
+    timestamp: Date.now(),
+  });
+}
+
+describe("estimateMessageCharsCached", () => {
+  it("returns a positive estimate for a valid message", () => {
+    const cache = createMessageCharEstimateCache();
+    const msg = makeUser("hello world");
+    expect(estimateMessageCharsCached(msg, cache)).toBeGreaterThan(0);
+  });
+
+  it("returns 0 for null", () => {
+    const cache = createMessageCharEstimateCache();
+    expect(estimateMessageCharsCached(null as unknown as AgentMessage, cache)).toBe(0);
+  });
+
+  it("returns 0 for undefined", () => {
+    const cache = createMessageCharEstimateCache();
+    expect(estimateMessageCharsCached(undefined as unknown as AgentMessage, cache)).toBe(0);
+  });
+
+  it("returns 0 for a non-object primitive", () => {
+    const cache = createMessageCharEstimateCache();
+    expect(estimateMessageCharsCached(42 as unknown as AgentMessage, cache)).toBe(0);
+  });
+
+  it("caches the estimate on second call", () => {
+    const cache = createMessageCharEstimateCache();
+    const msg = makeUser("cached test");
+    const first = estimateMessageCharsCached(msg, cache);
+    const second = estimateMessageCharsCached(msg, cache);
+    expect(first).toBe(second);
+    expect(first).toBeGreaterThan(0);
+  });
+});
+
+describe("estimateContextChars", () => {
+  it("sums estimates for valid messages", () => {
+    const cache = createMessageCharEstimateCache();
+    const messages = [makeUser("one"), makeUser("two")];
+    const total = estimateContextChars(messages, cache);
+    expect(total).toBeGreaterThan(0);
+  });
+
+  it("skips null entries without crashing", () => {
+    const cache = createMessageCharEstimateCache();
+    const messages = [
+      makeUser("valid"),
+      null as unknown as AgentMessage,
+      makeUser("also valid"),
+    ];
+    const total = estimateContextChars(messages, cache);
+    expect(total).toBeGreaterThan(0);
+  });
+
+  it("skips undefined entries without crashing", () => {
+    const cache = createMessageCharEstimateCache();
+    const messages = [
+      undefined as unknown as AgentMessage,
+      makeUser("valid"),
+    ];
+    const total = estimateContextChars(messages, cache);
+    expect(total).toBeGreaterThan(0);
+  });
+
+  it("handles an entirely null/undefined array", () => {
+    const cache = createMessageCharEstimateCache();
+    const messages = [
+      null as unknown as AgentMessage,
+      undefined as unknown as AgentMessage,
+    ];
+    expect(estimateContextChars(messages, cache)).toBe(0);
+  });
+
+  it("handles an empty array", () => {
+    const cache = createMessageCharEstimateCache();
+    expect(estimateContextChars([], cache)).toBe(0);
+  });
+});

src/agents/pi-embedded-runner/tool-result-char-estimator.ts

@@ -145,6 +145,9 @@ export function estimateMessageCharsCached(
   msg: AgentMessage,
   cache: MessageCharEstimateCache,
 ): number {
+  if (msg == null || typeof msg !== "object") {
+    return 0;
+  }
   const hit = cache.get(msg);
   if (hit !== undefined) {
     return hit;
@@ -158,7 +161,9 @@ export function estimateContextChars(
   messages: AgentMessage[],
   cache: MessageCharEstimateCache,
 ): number {
-  return messages.reduce((sum, msg) => sum + estimateMessageCharsCached(msg, cache), 0);
+  return messages
+    .filter((m): m is AgentMessage => m != null)
+    .reduce((sum, msg) => sum + estimateMessageCharsCached(msg, cache), 0);
 }
 
 export function invalidateMessageCharsCacheEntry(