ci: add Mergify merge queue config

[nobodyiam]

What's the purpose of this PR

Add a .mergify.yml for apollo-java so PR merge queue behavior is consistent with apolloconfig/apollo, while adapting conditions to this repository's main branch and current CI job names.

Which issue(s) this PR fixes:

Fixes #N/A (repository infrastructure improvement, no linked issue)

Brief changelog

• add .mergify.yml with single-commit (rebase) and multi-commit (squash) queue rules
• require successful checks for current apollo-java workflows/jobs (compile-matrix JDK matrix, integration/compat jobs, license, CLAssistant)
• add conflict notification comment rule for open PRs

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

Summary by CodeRabbit

• Chores
  • Configured automated merge queue management to optimize pull request processing and enforce consistent merge strategies based on commit requirements.

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request adds a Mergify configuration file that defines automated merge queue rules for single-commit and multi-commit pull requests with different merge strategies, along with a notification rule for PR conflicts.

Changes

Cohort / File(s)	Summary
Mergify Configuration .mergify.yml	Adds merge queue setup with max_parallel_checks=1, defines two queue rules (single-commit with rebase merge and multi-commit with squash merge), and a pull_request_rule to notify authors when conflicts exist.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 Hops of joy, the merge queue springs,
With Mergify's automated wings,
Single commits rebase clean,
Multi squash, a tidy scene,
Conflicts warned, the flow now sings! 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'ci: add Mergify merge queue config' accurately describes the main change—adding Mergify configuration for merge queue management. It is concise, specific, and clearly summarizes the primary modification.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.96%. Comparing base (d4b76f8) to head (5d7f53b).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #135      +/-   ##
============================================
+ Coverage     68.68%   70.96%   +2.28%     
- Complexity     1503     1632     +129     
============================================
  Files           212      224      +12     
  Lines          6396     6727     +331     
  Branches        647      679      +32     
============================================
+ Hits           4393     4774     +381     
+ Misses         1673     1601      -72     
- Partials        330      352      +22     

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[coderabbitai]

🧹 Nitpick comments (1)

.mergify.yml (1)

73-79: Duplicate conflict comments may be posted on repeated Mergify evaluations.

The comment action in Mergify's pull_request_rules does not deduplicate by default — it posts a new comment on every evaluation cycle where conflict and -closed are both true. If a PR stays conflicted across multiple pushes to the base branch, the author can receive multiple identical notifications.

A common mitigation is to add a label-gate: set a label on first notification and gate on its absence, clearing it when the conflict resolves.

✏️ Suggested label-gated conflict notification

+pull_request_rules:
+  - name: label PR when conflicts are detected
+    conditions:
+      - "conflict"
+      - "-closed"
+      - "-label=conflict"
+    actions:
+      label:
+        add:
+          - conflict
+      comment:
+        message: "@{{author}} This pull request has conflicts with the target branch. Please resolve them and update the branch before merging."
+
+  - name: remove conflict label when conflicts are resolved
+    conditions:
+      - "-conflict"
+      - "label=conflict"
+    actions:
+      label:
+        remove:
+          - conflict

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In @.mergify.yml around lines 73 - 79, The current Mergify rule "notify author
when PR has conflicts" posts duplicate comments because it runs every
evaluation; update the rule to gate on a label (e.g., "conflict-notified") being
absent and, in the rule's actions, add that label when posting the comment (use
add_label) so the comment is posted only once; also create a complementary rule
that matches when the PR no longer has the "conflict" condition but does have
the "conflict-notified" label and remove that label (use remove_label) so
notifications can be sent again if conflicts reappear.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In @.mergify.yml:
- Around line 73-79: The current Mergify rule "notify author when PR has
conflicts" posts duplicate comments because it runs every evaluation; update the
rule to gate on a label (e.g., "conflict-notified") being absent and, in the
rule's actions, add that label when posting the comment (use add_label) so the
comment is posted only once; also create a complementary rule that matches when
the PR no longer has the "conflict" condition but does have the
"conflict-notified" label and remove that label (use remove_label) so
notifications can be sent again if conflicts reappear.