AC-1305 chore: fix CI matrix (sqlite3 pin, drop unsupportable Rails edge)

Three separate causes, all observed in run 33625139653:

* Rails 6.1/7.0 on Ruby 3.0/3.1 (4 jobs): their sqlite3_adapter declares
  `gem "sqlite3", "~> 1.4"`, but the gemspec's sqlite3 dev dependency is
  unpinned, so Bundler resolved sqlite3 2.x -> "can't activate sqlite3
  (~> 1.4), already activated sqlite3-2.8.1". Ruby 2.7 passed only
  because sqlite3 2.x needs Ruby >= 3.0, so it got sqlite3 1.7.3.
  Fixed by pinning sqlite3 ~> 1.4 in those two gemfiles.

* Rails edge on Ruby 2.7/3.0/3.1 (3 jobs): rails main is 8.2.0.alpha,
  whose actionpack.gemspec requires Ruby >= 3.3.1, so Bundler cannot
  resolve on any Ruby in the matrix.

* Rails edge/7.2+ on any Ruby: test/helper.rb builds a bare RouteSet with
  no Rails app, and Rails 7.2 changed with_routing/reset_routes to reset
  through the app -> 25-27 errors ("undefined method `config' for nil").
  This needs a source change, not a matrix change, so edge is dropped
  rather than left permanently red.

Coverage goes from 9 jobs (2 green) to 10 jobs, adding Rails 7.1 on
Ruby 3.1-3.4. Rails 7.1 verified green locally on Ruby 3.3 and 3.4
(52 runs, 167 assertions, 0 failures, 0 errors).

Also bumps actions/checkout v2 -> v4 (hygiene; v2 is forced onto Node 24).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Daniel Wheeler

.github/workflows/ci.yml

@@ -6,15 +6,28 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
+      # Explicit combinations rather than a cross-product: each Rails version
+      # here supports a different Ruby range, and Rails 6.1/7.0 additionally
+      # require sqlite3 ~> 1.4 (pinned in their gemfiles), which is not
+      # installable on every Ruby in the matrix.
       matrix:
-        ruby: ['2.7', '3.0', '3.1']
-        rails: ['6.1', '7.0', 'edge']
+        include:
+          - { ruby: '2.7', rails: '6.1' }
+          - { ruby: '3.0', rails: '6.1' }
+          - { ruby: '3.1', rails: '6.1' }
+          - { ruby: '2.7', rails: '7.0' }
+          - { ruby: '3.0', rails: '7.0' }
+          - { ruby: '3.1', rails: '7.0' }
+          - { ruby: '3.1', rails: '7.1' }
+          - { ruby: '3.2', rails: '7.1' }
+          - { ruby: '3.3', rails: '7.1' }
+          - { ruby: '3.4', rails: '7.1' }
     env:
       BUNDLE_GEMFILE: gemfiles/rails_${{ matrix.rails }}.gemfile
     name: Ruby ${{ matrix.ruby }} / Rails ${{ matrix.rails }}
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with:

gemfiles/rails_6.1.gemfile

@@ -4,5 +4,7 @@ gem "actionpack", github: "rails/rails", branch: "6-1-stable"
 gem "activerecord", github: "rails/rails", branch: "6-1-stable"
 gem "railties", github: "rails/rails", branch: "6-1-stable"
 gem "rack", "< 3"
+# Rails 6.1's sqlite3_adapter declares `gem "sqlite3", "~> 1.4"`, so 2.x cannot be used.
+gem "sqlite3", "~> 1.4"
 
 gemspec :path => "../"

gemfiles/rails_7.0.gemfile

@@ -4,5 +4,7 @@ gem "actionpack", github: "rails/rails", branch: "7-0-stable"
 gem "activerecord", github: "rails/rails", branch: "7-0-stable"
 gem "railties", github: "rails/rails", branch: "7-0-stable"
 gem "rack", "< 3"
+# Rails 7.0's sqlite3_adapter declares `gem "sqlite3", "~> 1.4"`, so 2.x cannot be used.
+gem "sqlite3", "~> 1.4"
 
 gemspec :path => "../"

gemfiles/rails_7.1.gemfile

@@ -0,0 +1,9 @@
+source "https://rubygems.org"
+
+gem "actionpack", "~> 7.1.0"
+gem "activerecord", "~> 7.1.0"
+gem "railties", "~> 7.1.0"
+
+# Rails 7.1 relaxed its sqlite3 constraint, so no pin is needed here.
+
+gemspec :path => "../"