test(e2e): Update iOS envelope header test for Cocoa SDK v9 (#5623)

Updates envelope header trace expectations to match Cocoa SDK v9 format:
- Changed sampled field from "1" to "true" (per sentry-cocoa#3067)
- Added sample_rand field validation for Dynamic Sampling Context

This change is required after upgrading from Cocoa 8.58.0 to 9.3.0 on the v8 branch.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: antonis

samples/react-native/e2e/tests/captureHeader/envelopeHeader.test.ios.ts

@@ -60,8 +60,9 @@ describe('Capture message', () => {
           environment: expect.any(String),
           public_key: expect.any(String),
           replay_id: expect.any(String),
+          sample_rand: expect.any(String),
           sample_rate: '1',
-          sampled: '1',
+          sampled: 'true',
           trace_id: expect.any(String),
           transaction: 'ErrorsScreen',
         }),