Summary

Three EC2 bugs from the 2026-07-16 bug-hunt (all silent wrong-behavior):

• 1.1 (HIGH) DescribeVolumes infinite loop — it was the only one of EC2's five paginators missing the validate_max_results guard. MaxResults=0 produced an empty <volumeSet/> plus <nextToken>0</nextToken> pointing back at offset 0 → a paginating client loops forever. Now returns InvalidParameterValue like the four siblings (and like AWS).
• 4.1 (MED) change_state TOCTOU resurrection — Start/Stop/Terminate validated the transition + stop/termination protection under a read() guard, dropped it, then applied the new state under write() with no re-check. A concurrent TerminateInstances landing in the window let a StartInstances overwrite code 48 with pending, resurrecting a terminated instance past the boot task's terminal guard. Extracted check_transition and re-run it against freshly re-read state inside the write lock; the whole call is now atomic.
• 1.11 (LOW) empty filter value truncation — Filter.N.Value.M= (a legitimate empty/absent-tag-value filter) truncated the value list. Added indexed_list_keep_empty for filter values; id-lists (InstanceId.N etc.) keep the original empty-terminates semantics so there's no blast radius.

Test plan

• check_transition_enforces_terminal_and_protection (unit) + start_after_terminate_does_not_resurrect (tokio integration) — resurrection is rejected, state untouched.
• describe_volumes_rejects_max_results_zero / _above_max, describe_volumes_paginates.
• indexed_list_empty_value_terminates_for_id_lists, indexed_list_keep_empty_*, parse_filters_keeps_empty_filter_value.
• cargo test -p fakecloud-ec2 --lib → 132 passed; clippy --all-targets -D warnings clean; fmt applied.

Behavior-only fix — no API-surface, SDK, docs, or count changes.

Summary by cubic

Fixes three EC2 behavior bugs: prevents infinite pagination in DescribeVolumes, makes instance state changes atomic to close a TOCTOU gap, and preserves empty filter values during parsing. Behavior-only and aligns errors with AWS.

• Bug Fixes
  • DescribeVolumes: enforce MaxResults range (5–1000); 0 or >1000 now returns InvalidParameterValue instead of looping with a self-referential NextToken.
  • Start/Stop/Terminate: re-check transition and protections inside the write lock; prevents resurrection after a concurrent terminate; batch calls remain atomic (all-or-nothing).
  • Filters: keep empty filter values (Filter.N.Value.M=) using indexed_list_keep_empty; ID lists still treat empty members as terminators.

^{Written for commit 6865aec. Summary will update on new commits.}