Fix type safety for undefined xpath values (#863) (#865)

## Summary
- Add proper type annotations for xpath lookup that can return undefined
- Update trimTrailingTextNode to handle undefined input safely
- Fix potential runtime errors when xpath is not found in
combinedXpathMap

## Test plan
- [x] Verify no TypeScript errors

🤖 Generated with [Claude Code](https://claude.ai/code)

# why

# what changed

# test plan

---------

Co-authored-by: Jeff Barg <jeffbarg94@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

.changeset/bumpy-mugs-sneeze.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+improve type safety for trimTrailingTextNode

lib/handlers/observeHandler.ts

@@ -170,7 +170,7 @@ export class StagehandObserveHandler {
 
         if (elementId.includes("-")) {
           const lookUpIndex = elementId as EncodedId;
-          const xpath = combinedXpathMap[lookUpIndex];
+          const xpath: string | undefined = combinedXpathMap[lookUpIndex];
 
           const trimmedXpath = trimTrailingTextNode(xpath);
 

lib/utils.ts

@@ -493,6 +493,8 @@ export function loadApiKeyFromEnv(
   return undefined;
 }
 
-export function trimTrailingTextNode(path: string): string {
-  return path.replace(/\/text\(\)(\[\d+\])?$/iu, "");
+export function trimTrailingTextNode(
+  path: string | undefined,
+): string | undefined {
+  return path?.replace(/\/text\(\)(\[\d+\])?$/iu, "");
 }