Remove duo_non_agentic_chat_ai_message_cut_off_warning feature flag

What does this MR do and why?

Removes the duo_non_agentic_chat_ai_message_cut_off_warning feature flag. The feature (appending a warning to non-agentic Duo Chat responses when the stream is cut off without a finish_reason) has been validated and is ready to be enabled for all users.

Changes:

• Delete the feature flag YAML definition
• Remove the Feature.enabled? guard from StepExecutor#final_answer_streaming_cut_off?, making the cut-off warning always active
• Remove the feature-flag-disabled test context from the spec

References

• Rollout issue: #598963 (closed)
• Original feature issue: #583882 (closed)
• Introducing MR: !234238 (merged)

Screenshots or screen recordings

N/A — no UI changes.

How to set up and validate locally

1. Ask duo non agentic chat to generate large content, for example "print this entire file" with the user.rb model selected/open. You should see the cut off warning.

MR acceptance checklist

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