fix: strip leading whitespace from sanitizeUserFacingText output (#16158)

* fix: strip leading whitespace from sanitizeUserFacingText output

LLM responses frequently begin with \n\n, which survives through
sanitizeUserFacingText and reaches the channel as visible blank lines.

Root cause: the function used trimmed text for empty-checks but returned
the untrimmed 'stripped' variable. Two one-line fixes:
1. Return empty string (not whitespace-only 'stripped') for blank input
2. Apply trimStart() to the final return value

Fixes the same issue as #8052 and #10612 but at the root cause
(sanitizeUserFacingText) rather than scattering trimStart across
multiple delivery paths.

* Changelog: note sanitizeUserFacingText whitespace normalization

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/agents/pi-embedded-helpers.sanitizeuserfacingtext.e2e.test.ts

@@ -69,4 +69,25 @@ describe("sanitizeUserFacingText", () => {
     const text = "Hello there!\n\nDifferent line.";
     expect(sanitizeUserFacingText(text)).toBe(text);
   });
+
+  it("strips leading newlines from LLM output", () => {
+    expect(sanitizeUserFacingText("\n\nHello there!")).toBe("Hello there!");
+    expect(sanitizeUserFacingText("\nHello there!")).toBe("Hello there!");
+    expect(sanitizeUserFacingText("\n\n\nMultiple newlines")).toBe("Multiple newlines");
+  });
+
+  it("strips leading whitespace and newlines combined", () => {
+    expect(sanitizeUserFacingText("\n \n Hello")).toBe("Hello");
+    expect(sanitizeUserFacingText("  \n\nHello")).toBe("Hello");
+  });
+
+  it("preserves trailing whitespace and internal newlines", () => {
+    expect(sanitizeUserFacingText("Hello\n\nWorld\n")).toBe("Hello\n\nWorld\n");
+    expect(sanitizeUserFacingText("Line 1\nLine 2")).toBe("Line 1\nLine 2");
+  });
+
+  it("returns empty for whitespace-only input", () => {
+    expect(sanitizeUserFacingText("\n\n")).toBe("");
+    expect(sanitizeUserFacingText("  \n  ")).toBe("");
+  });
 });