TUI: rank prefix matches ahead of fuzzy results in slash command popup

[surajs-oai]

Summary:

• Show prefix-matching slash commands at the top of the popup, then append fuzzy matches.
• Keeps original command order while avoiding prefix/fuzzy collisions like /ps → /approvals.

Testing:

• cargo test -p codex-tui --quiet
• cargo test -p codex-tui2 --quiet

Issue: N/A

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[surajs-oai]

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[etraut-openai]

If we make this change here, we should probably make the same change in the Codex IDE extension. We're trying to keep the behaviors consistent across them.

[surajs-oai]

Addressed in #9629.