Pin kessai to the alpha that carries the claim record, now that one is released

Follow-up to !398 (merged), on the one finding that MR could not fix.

Pinning drupal/kessai was reverted there because ^1.0@alpha resolved to 1.0.0-alpha2, which predates the Claim entity added in [#3614985]. orchestra_payment's kernel tests install kessai_claim, so every payment class died with PluginNotFoundException: The "kessai_claim" entity type does not exist.

Kessai has since released 1.0.0-alpha4, which carries the claim entity, so the pin is now possible.

The constraint is ^1.0.0-alpha4@alpha, not ^1.0@alpha: alpha3 exists and was tagged at a commit before the claim record landed, so the looser constraint would still resolve to a Kessai the payment step cannot run against. Checked against the real tags:

Version	^1.0.0-alpha4@alpha
1.0.0-alpha2	no
1.0.0-alpha3	no
1.0.0-alpha4	match
1.1.0	match
2.0.0	no

The suggest text now names the claim record as the reason an earlier alpha will not do, so the constraint is not loosened back by someone tidying it.

The proof is the phpunit job: it installs what composer.json asks for, which a local kernel run never does (it uses the deployed contrib checkout, so it cannot see a wrong constraint at all).