#24676 follow-up: carry #24194 stable model menu fix

Carries Lucas Ribeiro’s #24194 model-menu stability improvement onto current feat-cmux-next. The follow-up preserves the compact menu height/width behavior and its tests while integrating the current base.

• Original PR: https://github.com/manaflow-ai/cmux/pull/24194
• Lucas remains the author of the carried commits; no unrelated changes.
• Validation: scoped diff 3 files, +257/-123; git diff --check clean.

agent-identity footer
Agent session: big-red

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

Carries Lucas Ribeiro's #24194 fix onto feat-cmux-next so the compact model menu no longer shifts while open: the menu holds the tallest tab's height and widest tab's width for the whole opening, so switching provider tabs or typing a filter no longer moves the rail out from under the pointer.

• A hidden spacer keeps the tallest tab's rows and separators, and a probe holds the widest tab's width.
• Tab, filter, and backspace changes scroll the list back to the top.
• The probe stays in sync with the drawn rows, keeping the account and fork rows intact.