Re-enable the Rails main CI job with rspec-rails main

[tricknotes]

Summary

The Rails main CI job was disabled because the released rspec-rails series mutates internals that Rails main freezes, failing the suite while loading the specs (FrozenError: can't modify frozen Hash). The fixes (e.g. rspec/rspec-rails#2915) are merged on rspec-rails main but not yet released.

• Widen the gemspec's rspec-rails development-dependency constraint from < 7.0 to < 9.0, admitting the 8.x series. The constraint also applies to the git source, whose current prerelease is 8.1.0.pre, so this is what lets the main branch resolve.
• When RAILS_VERSION=main, take rspec-rails from its main branch in the Gemfile, following the existing switch for the rails constraint.
• Re-enable the (4.0, main, 7.0.0) matrix entry, marked experimental with continue-on-error so the moving target can break without failing the build (the same arrangement as Use rspec-rails main when testing against Rails main ember-cli-rails-assets#36 relies on).

Notes

• With the widened constraint, released Rails versions now resolve rspec-rails to the released 8.x series (8.0.4 at the moment) instead of 6.1.5, so the whole matrix moves to rspec-rails 8 (which requires Rails >= 7.2 — the matrix's minimum).

Verification

• RAILS_VERSION=main bundle lock resolves, picking up rspec-rails 8.1.0.pre from main against rails main
• bundle lock with the default RAILS_VERSION resolves to rspec-rails 8.0.4
• spec/lib passes locally against rspec-rails 8.0.4 (79 of 81 examples; the 2 failures are a local environment issue — no ember-cli executable installed — not related to rspec-rails)
• Please rely on CI for the full matrix, and on the experimental build (4.0, main, 7.0.0) job for the Rails main run

🤖 Generated with Claude Code