[Bug] Fable 5.1 incorrectly blocked by usage credits prompt on Claude Max plan

**Bug Description**
Fable 5.1 blocked behind usage credits on Max 5x plan despite included Fable allowance. [/model prompt] "Switch to Fable 5.1? Fable 5.1 runs on usage credits, purchased separately from your plan. You don't have usage credits yet. Starts with a $20 monthly limit · run /usage-credits to adjust" — only options "No, keep my current model" / "Yes, buy usage credits". Chose No → "Kept model as Opus 5 (1M context) (default)". [/usage screen] Current session 13% used (resets 1:30pm Asia/Seoul); Current week (all models) 1% used, resets Sep 13 12pm; Current week (Fable) 2% used, resets Sep 13 12pm; Usage credits: off. [/status screen] Version 2.1.263; Login method: Claude Max account; Model: Default (Opus 5 with 1M context); Setting sources: User settings; OS Windows/PowerShell. [claude.ai Settings > Usage, same account] Plan: Max (5x); Current session 9%; Weekly — All models 1%, Fable 2%; banner: weekly Claude Code limit raised 50% until Sep 13. Per Help Center "Claude Fable models on your plan", Fable 5.1 is included on Max up to 50% of weekly limits, so the credits prompt is wrong — the client itself shows a Fable plan bucket at 2%. Same contradiction as issues #79412 / #79337 (Fable 5 on Max, 2026-07-20, resolved server-side). Already tried, all with same result: settings.json had "model":"fable[1m]" → removed line and restarted; selected standard Fable 5.1 entry (not 1M variant); /logout then /login and re-authorized Max account; usage credits kept OFF throughout. Observed 2026-09-07 (Asia/Seoul). Request: fix Fable 5.1 entitlement check for Max plan in Claude Code /model picker.

**Environment Info**
- Platform: win32
- Terminal: vscode
- Version: 2.1.263
- Feedback ID: 9f75eec7-21b3-42cb-bae2-c1bef0ee8e7b

**Errors**
```json
[]
```

Start by reproducing in Claude Code 2.1.263 with a Claude Max 5x account, using /model, /usage, and /status as described. Compare the Fable 5.1 entitlement path with the prior related issues #79412 and #79337. Done means the /model picker allows included Fable 5.1 usage on Max without requiring usage credits, while credits remain off.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.