ci: Notify the Go SDK validation failure and fix the release failure handler

[lantoli]

Proposed changes

Jira ticket: CLOUDP-458828

Adds a Slack notification to @apix-devops-integrations-help in #cloud-apix-devops-integrations-notifications when the go-sdk-validation step fails during an OpenAPI spec release, and fixes the release failure handler's duplicate-issue detection.

Notification:

• New Notify the Go SDK validation failure step in required-spec-validations.yml, keyed to steps.go-sdk-validation.outcome == 'failure'. A failure in an earlier setup step (Node, npm, artifact download, Spectral, Go, SDK checkout) leaves that outcome empty, so setup failures do not notify.
• Runs only on the first attempt after the automatic retries are exhausted (github.run_attempt == 3, the release retry cap in release-spec.yml), so the extra reruns the retry handler already queued do not duplicate the alert.
• New .github/scripts/notify_sdk_validation_failure.sh posts the environment, the run URL, and the help usergroup mention. It reports delivery failures and the case where the bot is not in the channel.
• The existing SLACK_BEARER_TOKEN secret is threaded explicitly through release-spec-runner.yml to release-spec.yml to required-spec-validations.yml.

Failure handler:

• Corrected the duplicate-issue query: (${TARGET_ENV}}) became (${TARGET_ENV}).
• Replaced return 0 / return 1 with exit 0 / exit 1. A run: step cannot return, so the step exited 2 and failed the job whenever an open failed-release issue already existed. That is what failed the handler on attempt 4 of run 36420941422.

Notes:

• The channel ID and usergroup ID are not sensitive, so they are inline; only the repository owner can manage repository secrets. They can move to a secret or variable if preferred.
• The bot must be a member of the notifications channel, and the usergroup mention must resolve.
• Verified locally: actionlint passes on the changed workflows and the new script parses with bash -n. ShellCheck is not installed locally; the shellcheck workflow will lint the new script.

Checklist

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works

Changes to Spectral

• I have read the README file for Spectral Updates

Further comments