fix: enforce allowed-account-ids on all auth paths (#1847)

* fix: enforce allowed-account-ids on all auth paths

The allowed-account-ids list was only enforced in some auth flows. This
was due to the check being included in validateCredentials, which was
skipped if (GITHUB_ACTIONS && AccessKeyId && output-env-credentials) ->
false.

This unifies credential validation into a single path.

- validateCredentials(credentials?, ...) resolves credentials, proves
  liveness via one GetCallerIdentity call, and returns the identity.
- validateAccountId(expectedAccountIds, account) is now a pure comparison
  against the resolved account, enforced against the final (assumed)
  account independent of auth method, GITHUB_ACTIONS, or
  output-env-credentials.
- exportAccountId(identity, ...) consumes the resolved identity instead of
  making its own GetCallerIdentity call, so credential resolution happens
  exactly once per credential set.

Pre-assume account checks remain gated on !roleToAssume so cross-account
assume-role (source account differs from the role's target) is preserved.

Adds regression tests for the OIDC wrong-account case (the previously
missing negative test), OIDC with output-env-credentials: false, and the
assume-role wrong-account case.

* chore: move validateAccountId into helpers

README.md

@@ -776,7 +776,10 @@ the environment (for example, on a self-hosted runner where you do not want the
 assumed-role credentials to shadow an existing EC2 instance profile), pair
 `output-credentials: true` with `output-env-credentials: false`. In that mode,
 the action does not run its post-credential SDK-pickup validation step, since
-the credentials were never written to the environment.
+the credentials were never written to the environment. The action still
+validates the resolved credentials by calling `sts:GetCallerIdentity` with the
+explicit credentials, so the `allowed-account-ids` check can be enforced if
+provided.
 
 ### Configure multiple AWS profiles in a single workflow