Describe FieldKernelTestBase::$fieldTestData keys to remove PHPStan baseline entries

Generated with the help of an LLM.

I asked an LLM to find easy to resolve issues added to the phpstan baseline, since we bumped to level 3. I then refined the fix and wrote this issue summary.

Problem/Motivation

There are 130 "Access to an undefined property ArrayObject" violations in the baseline. These are all due to loose typehints for \Drupal\Tests\field\Kernel\FieldKernelTestBase::$fieldTestData.

Steps to reproduce

List baseline entries:
grep -F 'Access to an undefined property ArrayObject' core/.phpstan-baseline.php

Sum them:
grep -F -A2 'Access to an undefined property ArrayObject' core/.phpstan-baseline.php | awk -F"=> " '/count/ {gsub(/,/,"",$2); s+=$2} END {print s}'

Proposed resolution

Document the expected keys / types for this property.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet