ci: trigger CI on merge_group so the merge queue can complete

[jstirnaman]

Summary

The default-branch ruleset requires the GitHub Merge Queue and requires this
workflow's Integration tests (InfluxDB 3 Core), Integration tests (InfluxDB Cloud Serverless), and Protocol tests jobs as status checks. The queue runs
checks against temporary gh-readonly-queue/main/* branches, which only
trigger workflows that opt in via the merge_group event. ci.yml only had
push and pull_request triggers, so no run ever started for queued
entries — they sat in AWAITING_CHECKS indefinitely, waiting on required
checks that could never execute. Confirmed via the GitHub API: zero Actions
runs exist on any gh-readonly-queue/* branch, and mergeQueue.entries sat
non-empty with every entry stuck at AWAITING_CHECKS.

• Add merge_group: to the workflow's on: triggers.
• Extend test-integration-cloud-serverless's forked-PR guard
  (if: github.event_name == 'push' || ...) to also allow
  github.event_name == 'merge_group'. Without this, the job would run on
  queue events but immediately evaluate its if to false and report
  skipped — leaving one of the three required checks permanently
  unsatisfied on queue runs, same symptom as before.

Related: the ruleset previously also had a required_deployments rule with
an empty environment list, which independently blocked the queue with
"Pull request cannot be added to a merge queue that requires deployments
before merging." That's already been removed from the ruleset directly
(no deployment workflow exists in this repo — the cloud-serverless
environment only scopes a test secret, which GitHub's UI mislabels as a
"deployment").

Test plan

• actionlint .github/workflows/ci.yml — clean
• zizmor .github/workflows/ci.yml — only pre-existing, unrelated
  excessive-permissions warnings
• Once merged, confirm the currently-queued PRs (chore(deps-dev): bump brace-expansion from 1.1.14 to 1.1.16 #72, chore(deps): bump body-parser from 2.2.2 to 2.3.0 #73, chore(deps): bump hono from 4.12.25 to 4.12.31 #76, chore(deps): bump fast-uri from 3.1.2 to 3.1.4 #77, chore(deps-dev): bump js-yaml from 4.2.0 to 4.3.0 #78,
  Add read-only ADR and 2026-07-23 e2e testbench results #79, Add tests for write-path error handling and packaging bugs #80) actually progress past AWAITING_CHECKS