ModelPrice prefix matching should sort by descending prefix length

[n0rvyn]

Symptom

ModelPrice.builtInDefaults comment says "ordered longest-prefix-first" but the array is not actually sorted by prefix length. Currently only the gpt-4o-mini / gpt-4o overlap is handled correctly by manual positioning. Future additions could introduce prefix shadowing bugs silently.

Prior Hypotheses

N/A — not a bug

Related Files

• ModelProxy/Models/ModelPrice.swift:21-41 — builtInDefaults array
• ModelProxy/Models/ModelPrice.swift:44-53 — lookup() uses .first with prefix matching

Notes

Two options:

1. Sort the array programmatically by descending prefix length at lookup time (or use a computed property)
2. Add a unit test that asserts no entry's prefix is a prefix of a later entry's prefix (catches shadowing at compile/test time)

Option 2 is lighter and catches the actual failure mode.