fix(core): Apply capture mechanism to originally captured exception - #23320

Fix custom exception mechanisms being applied to the deepest linked cause instead of the originally captured error.

LinkedErrors prepends causes before capture hints are applied, so the mechanism could overwrite a cause's chained metadata and incorrectly mark it as unhandled. Capture mechanisms now target the captured exception (identified by exception_id === 0 for exception groups) while preserving existing behavior for ordinary events.

Previous Behavior

In this scenario:

  const cause1 = new Error('Failure 1');
  const cause2 = new Error('Failure 2', { cause: cause1 });
  const error = new Error('Failure 3', { cause: cause2 });

  Sentry.captureException(error, {
    mechanism: { handled: false, type: 'auto.http.example' },
  });

...the mechanism of would be applied not to "Failure 3" but "Failure 1":

References

Also came up in this comment from 2025:

• https://github.com/getsentry/sentry-javascript/pull/17599/changes#r2341187673

Related to this PR (added tests to showcase problem):

• test(core): Keeping mechanism on captured error, not on its chained cause #23244