ci: enable GitHub merge queue

[Yicong-Huang]

What changes were proposed in this PR?

This PR enables GitHub merge queue for the default branch so ready PRs no longer need to repeatedly update against main after other PRs land.

Configuration changes:

• Add the required meta.environment: github_rulesets section for ASF-managed GitHub rulesets.
• Replace the previous protected_branches.main configuration with a default-branch ruleset.
• Keep the existing review gate: 1 approval, resolved review conversations, no code-owner requirement, and no last-push approval requirement.
• Keep the existing required checks: Required Checks, Check License Headers, and Validate PR title.
• Preserve GitHub auto-merge support.
• Disable the old strict branch-up-to-date policy so merge queue validates the final merge result instead of requiring every PR branch to be updated manually.
• Set the merge queue check timeout to 30 minutes because required checks are expected to finish in under 20 minutes, leaving buffer for runner startup or occasional slow runs.

Workflow changes:

• Add the merge_group event to required workflows so required checks run on merge queue refs.
• Keep PR title validation on pull_request_target; make the merge_group path a no-op because the PR title is validated before a PR enters the queue.

Expected behavior:

• Merge queue creates a temporary ref for main + queued PR(s) and runs required checks there.
• CI may run once before a PR enters the queue and once after it enters the queue.
• The second run protects main from integration failures in the queued merge result.
• If a merge-group run fails or times out, GitHub does not merge that group and the affected PR needs to be fixed/requeued.

Temporary auto-queue removal:

• Remove .github/workflows/auto-queue.yml because GitHub native merge queue replaces that scripted auto-queue behavior.
• Do not add any emergency label behavior in this PR. Emergency priority should be handled manually through GitHub queue controls or admin bypass if truly necessary.

Any related issues, documentation, discussions?

Closes #4553, also discussed in https://issues.apache.org/jira/browse/INFRA-27864

How was this PR tested?

Ran local configuration checks:

ruby -e 'require "yaml"; [".asf.yaml", ".github/workflows/required-checks.yml", ".github/workflows/check-header.yml", ".github/workflows/lint-pr.yml"].each { |f| YAML.load_file(f); puts "ok #{f}" }'
ruby -e 'require "yaml"; cfg=YAML.load_file(".asf.yaml"); abort "missing meta" unless cfg["meta"] && cfg["meta"]["environment"] == "github_rulesets"; mq=cfg["github"]["rulesets"].find { |r| r["name"] == "Merge Queue" }; abort "missing merge queue" unless mq; abort "main branch protection still present" if cfg.dig("github", "protected_branches", "main"); checks=mq["rules"].find { |r| r["type"] == "required_status_checks" }["parameters"]["required_status_checks"].map { |c| c["context"] }; abort checks.inspect unless checks == ["Required Checks", "Check License Headers", "Validate PR title"]; puts "asf ruleset sanity ok"'
git diff --check

No automated runtime tests were added because this PR only changes repository/GitHub Actions configuration.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: OpenAI Codex (GPT-5)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.78%. Comparing base (078ab64) to head (267c2d1).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5036   +/-   ##
=========================================
  Coverage     42.77%   42.78%           
+ Complexity     2195     2194    -1     
=========================================
  Files          1045     1045           
  Lines         39985    39985           
  Branches       4217     4217           
=========================================
+ Hits          17105    17107    +2     
+ Misses        21820    21819    -1     
+ Partials       1060     1059    -1     

Flag	Coverage Δ
access-control-service	39.53% <ø> (ø)
agent-service	33.72% <ø> (ø)
amber	43.50% <ø> (+0.01%)	⬆️
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <ø> (ø)
file-service	32.18% <ø> (ø)
frontend	34.04% <ø> (ø)
python	88.90% <ø> (ø)
workflow-compiling-service	47.72% <ø> (ø)

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