✨[RUM-15126] Allow updates of error.handling_stack

[bcaudan]

Motivation

Similarly to RUM errors error.stack attribute, allow error.handling_stack to be updated by customers before being sent.

Changes

• Update rum-events-format
• Add error.handling_stack to error modifiable fields
• Remove sandbox DD_FLAGGING global

Test instructions

Updating the value through beforeSend:

{
  beforeSend: (event) => {
    if (event.error?.handling_stack) {
      event.error.handling_stack = 'test'
    }
  }
},

And sending a custom error:

DD_RUM.addError('test')

The handling_stack value should be updated:

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-datadog-prod-us1-2]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.29% (-0.01%)

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