Fix gitlab_release without ref and when deleted outside of Terraform

What does this MR do and why?

Two fixes in gitlab_release:

1. The releases API never returns ref, and the provider never set it, so a release created or updated without ref (for a tag that already exists) failed the apply with Provider returned invalid result object after apply, after GitLab had created the release. ref stays computed and is now set from the plan: a configured ref is kept as before, and one that isn't configured is stored as an empty string.
2. Read returned the 404 of a release deleted outside of Terraform as an error, so every plan failed with Unable to get a release: 404 Not Found. It now removes the release from the state, so the plan creates it again. Delete treats a release that is already gone as deleted, as !3334 (merged) did for three integrations. GitLab answers the deletion of a missing release with 403 rather than 404, since it authorizes against the release before looking for it, so Delete takes a 403 as gone only when reading the release answers 404.

Closes #6919 (closed)

How to verify

Against a local make testacc-up instance:

unset GITLAB_TOKEN && make testacc RUN=TestAcc_GitlabRelease_

On main, the four new tests fail: TestAcc_GitlabRelease_existingTagWithoutRef with the unknown ref, TestAcc_GitlabRelease_deletedOutsideTerraform with the 404 in Read, and TestAcc_GitlabRelease_destroyDeletedRelease and TestAcc_GitlabRelease_destroyAfterProjectDeleted, whose destroys run without a refresh, with the 403 and the 404 in Delete. With the branch the seven tests pass.

What I ran (GitLab CE 19.5.0, Go 1.26.10, Terraform 1.4.0, golangci-lint 2.15.0)

The acceptance tests ran against a GitLab CE 19.5.0 container set up as make testacc-up sets it up, with Puma held to 4 workers to fit the host's memory.

--- PASS: TestAcc_GitlabRelease_destroyAfterProjectDeleted (2.75s)
--- PASS: TestAcc_GitlabRelease_deletedOutsideTerraform (3.31s)
--- PASS: TestAcc_GitlabRelease_destroyDeletedRelease (3.68s)
--- PASS: TestAcc_GitlabRelease_existingTagWithoutRef (4.05s)
--- PASS: TestAcc_GitlabRelease_releasedAt (4.34s)
--- PASS: TestAcc_GitlabRelease_milestones (3.35s)
--- PASS: TestAcc_GitlabRelease_basic (6.38s)
PASS

With main's resource_gitlab_release.go the four new tests fail and the three older ones pass. Without the line that sets ref, TestAcc_GitlabRelease_existingTagWithoutRef fails with the unknown ref. With the branch merged into main, the seven tests pass.

On the head, go mod tidy and tfplugindocs generate change no file, gofmt -s and go vet -tags acceptance ./internal/provider/... are clean, make lint-golangci reports 0 issues, the lint-tfprovider, lint-examples-sh, lint-examples-tf and lint-custom checks pass, and make test passes.

This work comes from maintaining gitlab-mcp-server, whose upstream-bugs.md records everything done on its dependencies and sibling projects.