[Specs][OpenAPI] Stabilize builder title specs across checkout names

[RedZapdos123]

Description:

Issue #345 reported that the mocked OpenAPI builder specs hardcode "title" => "Rage" while the current default title is derived from Rage.root.basename.

This PR keeps the current runtime behavior unchanged and stabilizes the mocked OpenAPI builder specs by stubbing Rage.root to a fixed rage path in the shared mocked_rage_routes test context.

That keeps the builder specs independent of the local checkout directory basename without changing Rage::OpenAPI::Converter.

It also adds the matching changelog entry required by the repository workflow.

Closes #345.

Before fix:

$ bundle exec ruby /mnt/c/Users/Xeron/Desktop/FOSS/FOSSContributons20/.tmp/openapi-title-drift-evidence/title_repro.rb
default_title: Rage Openapi Title Drift 20260702

$ bundle exec rspec spec/openapi/builder/base_spec.rb:16 --format documentation
Run options: include {:locations=>{"./spec/openapi/builder/base_spec.rb"=>[16]}}

Rage::OpenAPI::Builder
  with no routes
    returns correct schema (FAILED - 1)

expected: {"components"=>{}, "info"=>{"title"=>"Rage", "version"=>"1.0.0"}, "openapi"=>"3.0.0", "paths"=>{}, "tags"=>[]}
got:      {"components"=>{}, "info"=>{"title"=>"Rage Openapi Title Drift 20260702", "version"=>"1.0.0"}, "openapi"=>"3.0.0", "paths"=>{}, "tags"=>[]}

1 example, 1 failure

After fix:

$ bundle exec ruby /mnt/c/Users/Xeron/Desktop/FOSS/FOSSContributons20/.tmp/openapi-title-drift-evidence/title_repro.rb
default_title: Rage Openapi Title Drift 20260702

$ bundle exec rspec spec/openapi/builder/base_spec.rb:16 --format documentation
Run options: include {:locations=>{"./spec/openapi/builder/base_spec.rb"=>[16]}}

Rage::OpenAPI::Builder
  with no routes
    returns correct schema

1 example, 0 failures

$ bundle exec rspec spec/openapi
427 examples, 0 failures, 1 pending

Checklist:

• I have updated CHANGELOG.md.
• I have verified the runtime default title is unchanged in a non-rage checkout using bundle exec ruby /mnt/c/Users/Xeron/Desktop/FOSS/FOSSContributons20/.tmp/openapi-title-drift-evidence/title_repro.rb.
• I have run focused tests in WSL using bundle exec rspec spec/openapi/builder/base_spec.rb:16 --format documentation.
• I have run OpenAPI specs in WSL using bundle exec rspec spec/openapi.