Upgrade dev container to Ruby 4.0.4

[yahonda]

Summary

• Bump .devcontainer/devcontainer.json Ruby feature pin from 4.0.2 to 4.0.4.
• Ruby 4.0.4 is available in ghcr.io/rails/devcontainer/features/ruby:2 since rails/devcontainer#126.
• CI matrices ('4.0' floating constraint) and CONTRIBUTING.md ("Ruby 4.0") already track the major.minor and need no edits.

Test plan

• Devcontainer rebuild picks up Ruby 4.0.4
• ruby -v inside the rebuilt container reports ruby 4.0.4
• CI green

[yahonda]

Confirmed.

vscode ➜ /workspaces/oracle-enhanced $ ruby -v
ruby 4.0.4 (2026-05-12 revision b89eb1bcbf) +PRISM [aarch64-linux]
vscode ➜ /workspaces/oracle-enhanced $