test(e2e): prefix all API test resource names with cli-e2e-

[natalie-o-perret]

Description

The CLI's API e2e tests share an Exoscale test organisation with other repos (terraform-provider-exoscale, csi-driver, ...). Two scenarios creating a resource with the same name would race and one would fail with already exists or worse, leak.

This PR makes the CLI's e2e tests use a namespaced prefix for every resource they create. The prefix cli-e2e- is now baked into the injected TEST_RUN_ID by the runner, and existing literal cli-e2e- segments in scenario files have been removed so the convention lives in exactly one place.

Result: a resource like an NLB is now created as cli-e2e-1717000000-abc123-nlb-a instead of the previous mix (e2e-...-nlb-a, cli-e2e-rdns-..., cli-e2e-pg-...).

Related issue

N/A, came out of a chat about shared test org resource collisions.

Changes

• tests/e2e/testscript_api_test.go: RunID is now formatted as cli-e2e-<unix>-<rand6> instead of e2e-<unix>-<rand6>. One line, plus a short comment.
• tests/e2e/scenarios/with-api/compute/instance_reboot.txtar: dropped literal cli-e2e-, now reboot-$TEST_RUN_ID.
• tests/e2e/scenarios/with-api/compute/instance_create_reverse_dns.txtar: same, now rdns-$TEST_RUN_ID.
• tests/e2e/scenarios/with-api/dbaas/pg_config_lifecycle.txtar: same, now pg-$TEST_RUN_ID (6 occurrences).
• tests/e2e/README.md: new quick reference, documents the cli-e2e- prefix rule with good and bad naming examples.
• CHANGELOG.md: entry under Unreleased.

The NLB scenario already used $TEST_RUN_ID in its resource names, so no change was needed there. The nonexistent-e2e-instance placeholder in the not-found error scenarios is intentionally not a real resource and is unchanged.

Checklist

• Changelog updated (Unreleased block)
• Testing

Testing

• go vet ./... and go build ./... from tests/e2e/ pass on the default build tag.
• go vet -tags=api ./... and go build -tags=api ./... from tests/e2e/ also pass.

I have not been able to run the actual -tags=api scenarios here because they need EXOSCALE_API_KEY and EXOSCALE_API_SECRET. CI on this PR should be the source of truth for the full run.

Notes for reviewers

The single source of truth for the prefix is the runID line in testscript_api_test.go. Anyone adding a new with-api scenario should use <descriptor>-$TEST_RUN_ID and let the runner add cli-e2e-. The tests/e2e/README.md quick reference calls this out.