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/errors.ts

@@ -488,7 +488,7 @@ export function sanitizeUserFacingText(text: string, opts?: { errorContext?: boo
   const stripped = stripFinalTagsFromText(text);
   const trimmed = stripped.trim();
   if (!trimmed) {
-    return stripped;
+    return "";
   }
 
   // Only apply error-pattern rewrites when the caller knows this text is an error payload.
@@ -527,7 +527,7 @@ export function sanitizeUserFacingText(text: string, opts?: { errorContext?: boo
     }
   }
 
-  return collapseConsecutiveDuplicateBlocks(stripped);
+  return collapseConsecutiveDuplicateBlocks(stripped).trimStart();
 }
 
 export function isRateLimitAssistantError(msg: AssistantMessage | undefined): boolean {