[chore]: strengthen regex validation on act, observe for elementId (#1614)

# why
Stricter validation on LLM responses for chosen elementId to interact
with

# what changed
V3 includes **only** elementIds in the shape `number-number` to account
for iframe ordinals. This PR ensures the LLMs have stricter validation
on the elementId generation by changing the observe and act schemas to
to enforce
```typescript
z.string().regex(/^\d+-\d+$/)
```

# test plan
- [x] act evals
- [x] observe evals
- [x] combination evals
- [x] regression evals

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Enforce 'number-number' format for elementId in observe and act via
regex (^\d+-\d+$) to prevent invalid IDs. Aligns with Linear STG-675
guardrail requirements for elementId formatting.

<sup>Written for commit 3b70cc1.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1614">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

Author: miguelg719

.changeset/clean-birds-fetch.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+Enforce <number>-<number> regex validation on act/observe for elementId

packages/core/lib/inference.ts

@@ -245,6 +245,7 @@ export async function observe({
         z.object({
           elementId: z
             .string()
+            .regex(/^\d+-\d+$/)
             .describe(
               "the ID string associated with the element. Never include surrounding square brackets. This field must follow the format of 'number-number'.",
             ),
@@ -382,6 +383,7 @@ export async function act({
   const actSchema = z.object({
     elementId: z
       .string()
+      .regex(/^\d+-\d+$/)
       .describe(
         "the ID string associated with the element. Never include surrounding square brackets. This field must follow the format of 'number-number'.",
       ),