Simplify e2e diff validation test

- Scenario 0: Use auto-delete branch setting to trigger GitHub's
  auto-retarget naturally, instead of manual retarget
- Scenario 0: Reduce to 2 PRs (3rd was unnecessary)
- Scenario 0: Use retry loop instead of arbitrary sleep
- Scenario 0: Remove redundant pollution checks (identity check suffices)
- Remove scenario 0b: merge diff validation into scenario 1
- Scenario 1: Add diff preservation check (compare before/after)
- Add wait_for_pr_base_change helper function

Author: claude