Split rspec-package-tests into parallel generator/unit shards (#3134)

## Summary
- Splits the `rspec-package-tests` CI job into two parallel matrix
shards: **generators** (429 examples with heavy filesystem I/O) and
**unit** (584 examples covering everything else)
- Applies to both PR (latest-only) and main (latest + minimum) matrix
configurations
- Artifact names updated to include shard name to avoid collisions

Expected CI time reduction: ~24 min → ~12 min.

Closes #3131

## Test plan
- [ ] Verify both `generators` and `unit` shard jobs appear in the CI
matrix
- [ ] Confirm generator shard runs only `spec/react_on_rails/generators`
- [ ] Confirm unit shard runs `spec/react_on_rails` excluding generators
- [ ] Check artifact names are unique per shard
- [ ] Verify full matrix on main includes 4 jobs (2 Ruby versions × 2
shards)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Medium risk because it changes CI execution by splitting the RSpec
suite into shard-specific commands and adjusts test initialization to
suppress Minitest/Test::Unit autorun behavior, which could affect what
tests run and how failures surface.
> 
> **Overview**
> Splits `rspec-package-tests` into two parallel matrix shards
(`generators` vs `unit`) for both PR and full (main/`full-ci`) runs, and
runs shard-specific RSpec commands (generators-only vs everything
excluding generators).
> 
> Updates uploaded artifact names to include the shard to avoid
collisions, and tweaks both `spec_helper.rb` files to `require
"minitest"` and disable Minitest/Test::Unit autorun hooks so RSpec CLI
args don’t get parsed by Minitest during CI/test boot.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
7b8a9c8. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Updated CI to run tests in parallel shards (generators and unit),
improving feedback speed and artifact reporting.
* **Bug Fixes**
* Stabilized test boot process to prevent interfering test runners
during spec runs, improving test reliability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: justin808

.github/workflows/gem-tests.yml

@@ -86,13 +86,16 @@ jobs:
       - id: set-matrix
         run: |
           # Determine if we should run full matrix (main or full-ci label)
+          # Each Ruby version / dependency level is split into two shards:
+          #   generators – spec/react_on_rails/generators (heavy filesystem I/O)
+          #   unit       – everything else under spec/react_on_rails
           if [[ "${{ github.ref }}" == "refs/heads/main" ]] || \
              [[ "${{ needs.detect-changes.outputs.has_full_ci_label }}" == "true" ]]; then
-            # Full matrix: test both latest and minimum supported versions
-            echo 'matrix={"include":[{"ruby-version":"3.4","dependency-level":"latest"},{"ruby-version":"3.2","dependency-level":"minimum"}]}' >> $GITHUB_OUTPUT
+            # Full matrix: test both latest and minimum supported versions × 2 shards
+            echo 'matrix={"include":[{"ruby-version":"3.4","dependency-level":"latest","shard":"generators"},{"ruby-version":"3.4","dependency-level":"latest","shard":"unit"},{"ruby-version":"3.2","dependency-level":"minimum","shard":"generators"},{"ruby-version":"3.2","dependency-level":"minimum","shard":"unit"}]}' >> $GITHUB_OUTPUT
           else
-            # PR matrix: test only latest versions for fast feedback
-            echo 'matrix={"include":[{"ruby-version":"3.4","dependency-level":"latest"}]}' >> $GITHUB_OUTPUT
+            # PR matrix: test only latest versions for fast feedback × 2 shards
+            echo 'matrix={"include":[{"ruby-version":"3.4","dependency-level":"latest","shard":"generators"},{"ruby-version":"3.4","dependency-level":"latest","shard":"unit"}]}' >> $GITHUB_OUTPUT
           fi
 
   rspec-package-tests:
@@ -152,14 +155,22 @@ jobs:
         run: |
           echo "CI_DEPENDENCY_LEVEL=${{ matrix.dependency-level }}" >> $GITHUB_ENV
       - name: Run rspec tests
-        run: cd react_on_rails && bundle exec rspec spec/react_on_rails
+        env:
+          SHARD: ${{ matrix.shard }}
+        run: |
+          cd react_on_rails
+          if [[ "$SHARD" = "generators" ]]; then
+            bundle exec rspec spec/react_on_rails/generators
+          else
+            bundle exec rspec spec/react_on_rails --exclude-pattern "**/generators/**"
+          fi
       - name: Store test results
         uses: actions/upload-artifact@v4
         with:
-          name: main-rspec-${{ github.run_id }}-${{ github.job }}-ruby${{ matrix.ruby-version }}-${{ matrix.dependency-level }}
+          name: main-rspec-${{ github.run_id }}-${{ github.job }}-ruby${{ matrix.ruby-version }}-${{ matrix.dependency-level }}-${{ matrix.shard }}
           path: ~/rspec
       - name: Store artifacts
         uses: actions/upload-artifact@v4
         with:
-          name: main-test-log-${{ github.run_id }}-${{ github.job }}-ruby${{ matrix.ruby-version }}-${{ matrix.dependency-level }}
+          name: main-test-log-${{ github.run_id }}-${{ github.job }}-ruby${{ matrix.ruby-version }}-${{ matrix.dependency-level }}-${{ matrix.shard }}
           path: log/test.log

react_on_rails/spec/react_on_rails/spec_helper.rb

@@ -14,14 +14,22 @@
 require "rspec/retry"
 
 require "action_controller"
+require "minitest"
 
 require "rails"
+
+# Prevent Minitest's at_exit runner from parsing RSpec-only CLI args after
+# rails/test_help loads ActiveSupport's autorun hooks.
+# rubocop:disable Style/ClassVars
+Minitest.class_variable_set(:@@installed_at_exit, true) if Minitest.class_variable_defined?(:@@installed_at_exit)
+# rubocop:enable Style/ClassVars
+
 require "rails/test_help"
 Rails.backtrace_cleaner.remove_silencers!
 
 require_relative "simplecov_helper"
 
-# prevent Test::Unit's AutoRunner from executing during RSpec's rake task
+# Prevent Test::Unit's AutoRunner from executing during RSpec runs.
 Test::Unit.run = true if defined?(Test::Unit) && Test::Unit.respond_to?(:run=)
 
 # This file was generated by the `rails generate rspec:install` command. Conventionally, all

react_on_rails_pro/spec/react_on_rails_pro/spec_helper.rb

@@ -11,12 +11,20 @@
 require "pry-byebug"
 
 require "action_controller"
+require "minitest"
 require "rails"
+
+# Prevent Minitest's at_exit runner from parsing RSpec-only CLI args after
+# rails/test_help loads ActiveSupport's autorun hooks.
+# rubocop:disable Style/ClassVars
+Minitest.class_variable_set(:@@installed_at_exit, true) if Minitest.class_variable_defined?(:@@installed_at_exit)
+# rubocop:enable Style/ClassVars
+
 require "rails/test_help"
 Rails.backtrace_cleaner.remove_silencers!
 
 require_relative "simplecov_helper"
-# prevent Test::Unit's AutoRunner from executing during RSpec's rake task
+# Prevent Test::Unit's AutoRunner from executing during RSpec runs.
 Test::Unit.run = true if defined?(Test::Unit) && Test::Unit.respond_to?(:run=)
 
 require "shakapacker"