Remove orbit_foundational_agent feature flag

What does this MR do and why?

Removes the orbit_foundational_agent feature flag. It has been default_enabled: true since 19.2, so this keeps the enabled path and drops the disabled one, along with the specs that covered it.

• Delete the flag definition.
• Ai::Orbit::Settings.platform_available? now only checks that the Knowledge Graph is enabled for the user.
• Replace the flag with gon.orbit_available (Analytics::KnowledgeGraph.service_configured?, the same condition the flag was pushed under). orbit_toggle.vue and duo_agentic_chat_state_manager_light.vue skip the preference query and hide the toggle when it is false.
• Remove specs that stubbed the flag off and add coverage for the not-configured path.

References

Relates to #597629 (closed)

How to set up and validate locally

1. Run the updated specs under ee/spec/models/ai/orbit/settings_spec.rb, ee/spec/services/ai/duo_workflows/mcp_config_service_spec.rb and the two Orbit frontend specs.

Agent context

• gon_helper_spec.rb has 17 failures from LoadError: cannot load such file -- sass (Vite helper) on unmodified master in this environment. The new #push_orbit_availability examples pass.
• Jest: orbit_toggle_spec.js and duo_agentic_chat_state_manager_light_spec.js pass (86 tests) with the built fe_islands dist linked in.
• Overlaps with the in-flight Orbit UI redesign MR !259468 (closed), which edits the same comment block in settings.rb and still mentions the flag. Whichever lands second needs to drop the flag mention.
• info_service_spec.rb used the flag name only as a sample orbit_-prefixed flag, so it now uses orbit_trial.