fix(acp): honor config.json model when provider not yet loaded

[chaodu-agent]

Summary

When defaultModel() finds a model specified in config.json but the corresponding provider is not in the providers list, return the specified model instead of falling through to Provider.sort().

Current Behavior

config.json: {"model": "mimo/mimo-auto"}
providers:   [xiaomi]  (mimo not yet indexed)

defaultModel():
  ├─ config.json → specified = "mimo/mimo-auto"          ✅
  ├─ providers.find("mimo") → NOT FOUND
  ├─ (specified exists BUT providers.length > 0)
  │   └─ does NOT return specified
  ├─ falls through ↓
  ├─ Provider.sort([xiaomi models...])
  │   └─ picks "xiaomi/mimo-v2.5-pro-ultraspeed"         ❌ paid
  └─ calls paid API → 0 tokens → "(no response)"

Expected Behavior

config.json: {"model": "mimo/mimo-auto"}
providers:   [xiaomi]  (mimo not yet indexed)

defaultModel():
  ├─ config.json → specified = "mimo/mimo-auto"          ✅
  ├─ providers.find("mimo") → NOT FOUND
  ├─ (specified exists, provider missing)
  │   └─ RETURNS specified  ← trust user's explicit config
  └─ calls mimo-auto API → works                         ✅

Fix

When the specified provider is not found in the providers list, return the user's choice rather than falling through to Provider.sort() which picks an unintended paid model.

Fixes #865

[chaodu-agent]

Superseded by #1172 (combined with #1171, added unit tests)