fix(cloudformation): report ChangeSetNotFound for an unknown change set

[Sorttech]

cdk deploy and cdk bootstrap hung forever against a stack that
already exists.

Before creating its change set the CDK CLI deletes any leftover one with
the same name, then polls DescribeChangeSet until the call reports
DELETE_COMPLETE/DELETE_FAILED or raises ChangeSetNotFoundException
(waitForGone). On a lookup miss DescribeChangeSet fabricated
{"Status": "CREATE_COMPLETE", "ExecutionStatus": "AVAILABLE"}, so the
change set was never gone and the poll never terminated: one
DeleteChangeSet followed by DescribeChangeSet every five seconds,
with CreateChangeSet never reached.

ChangeSetNotFoundException is declared on DescribeChangeSet,
DescribeChangeSetHooks and ExecuteChangeSet, with awsQueryError code
ChangeSetNotFound and HTTP 404 (aws-models/cloudformation.json), so
returning it costs no conformance. DeleteChangeSet does not declare it
and still succeeds for a change set that was never created, matching AWS.

All three reads had the same fabricated success with the lookup
copy-pasted, ExecuteChangeSet explicitly so ("pass-through success
rather than hard-fail"). They now share one find_change_set and map a
miss to the error, which is also what CONTRIBUTING's no-stub-responses
rule asks for.

The unit coverage was asserting the stub: change_sets built a fresh
service per call, so DescribeChangeSet("cs") only passed because the
miss was faked. It now runs the lifecycle against one service.

Test plan

Regression test: describe_change_set_reports_unknown_change_set_as_not_found in crates/fakecloud-e2e/tests/cloudformation_execute_change_set.rs, plus the change_sets unit lifecycle in extras.rs which previously asserted the stub

Verification

On this exact head, rebased onto current main:

• cargo fmt --all --check - clean.
• cargo clippy --workspace --all-targets -- -D warnings - clean.
• cargo test --workspace excluding fakecloud-e2e, fakecloud-conformance,
  fakecloud-tfacc and fakecloud-parity (the same set CI's test job runs),
  plus cargo test -p fakecloud-conformance --lib - 9929 passed, 0 failed.
• cargo test -p fakecloud-e2e --test cloudformation_execute_change_set - the
  regression test above passes against a freshly built target/debug/fakecloud.

Found by deploying a CDK CloudFront + S3 SPA against fakecloud.

[Sorttech]

Retargeted upstream as faiscadev#2563.