feat(tool-truncation): use head+tail strategy to preserve errors during truncation (#20076)

Merged via squash.

Prepared head SHA: 6edebf22b1
Co-authored-by: jlwestsr <52389+jlwestsr@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

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

@@ -289,3 +289,25 @@ describe("sessionLikelyHasOversizedToolResults", () => {
     ).toBe(false);
   });
 });
+
+describe("truncateToolResultText head+tail strategy", () => {
+  it("preserves error content at the tail when present", () => {
+    const head = "Line 1\n".repeat(500);
+    const middle = "data data data\n".repeat(500);
+    const tail = "\nError: something failed\nStack trace: at foo.ts:42\n";
+    const text = head + middle + tail;
+    const result = truncateToolResultText(text, 5000);
+    // Should contain both the beginning and the error at the end
+    expect(result).toContain("Line 1");
+    expect(result).toContain("Error: something failed");
+    expect(result).toContain("middle content omitted");
+  });
+
+  it("uses simple head truncation when tail has no important content", () => {
+    const text = "normal line\n".repeat(1000);
+    const result = truncateToolResultText(text, 5000);
+    expect(result).toContain("normal line");
+    expect(result).not.toContain("middle content omitted");
+    expect(result).toContain("truncated");
+  });
+});