ci: test merge queue support - #1

## Summary

- run Continuous Integration and Govulncheck for GitHub merge-group
commits
- ensure Validate Server Version executes for merge groups instead of
being skipped by its PR-specific condition
- verify the root and `cliext` Go modules remain tidy
- remove two stale `ui-server/v2 v2.50.1` checksums exposed by the
initial tidy run

## Why

GitHub merge queues test synthetic `merge_group` commits. Required
workflows that only listen for `pull_request` do not create checks for
those commits, leaving queued pull requests blocked until timeout.

The server-version job also used `github.base_ref`, which is populated
for pull requests but not merge groups. Explicitly accepting the
`merge_group` event ensures the validation actually runs on the queued
commit.

The tidy check provides additional protection for dependency updates
that merge without a textual conflict but leave module metadata
inconsistent.

## Validation
- end-to-end queue test in `chaptersix-org/temporal-cli`: GitHub created
a `gh-readonly-queue/main/...` ref, triggered both workflows, ran
Validate Server Version, and merged the test PR through the queue

Sandbox evidence:

- chaptersix-org#1
-
https://github.com/chaptersix-org/temporal-cli/actions/runs/29164112024
-
https://github.com/chaptersix-org/temporal-cli/actions/runs/29164112020