fix(oauth): extraUsage always null — parseManagedUsagePayload reads "boosterWallet" but /usages returns "booster_wallet"

Bug

GET https://api.kimi.com/coding/v1/usages returns the booster wallet under the
snake_case key booster_wallet, but parseManagedUsagePayload reads the camelCase
key boosterWallet. As a result, quota.extraUsage is always null in
production, even for accounts with an active booster wallet and remaining balance.

Introduced by #3787 (bb0c66a1, released in 0.43.0), still present on latest main.

Evidence

Live response from GET /usages (2026-09-16, redacted):

{
  "usages": { "limit_5h": {}, "limit_7d": {} },
  "booster_wallet": {
    "balance": { "type": "BOOSTER", "amount": "20000000000",
                 "amountLeft": "10336703900", "unit": "UNIT_CURRENCY" },
    "status": "STATUS_ACTIVE",
    "monthlyUsed": { "currency": "CNY", "priceInCents": "9663" }
  }
}

But packages/oauth/src/managed-usage.ts:

return {
  usages: parseQuotaUsages(payload['usages']),
  extraUsage: parseBoosterWallet(payload['boosterWallet']), // <- never matches
};

Note the sibling usages parsing correctly uses snake_case (limit_5h,
limit_7d, used_ratio, reset_time) — only the booster wallet key is camelCase.

Why the tests did not catch it

• packages/oauth/test/managed-usage.test.ts feeds parseManagedUsagePayload a
  fixture with the camelCase key boosterWallet, matching the buggy reader instead
  of the real server payload.
• The fetchManagedUsage test's mocked /usages response contains no booster
  wallet at all, so the wire format is never exercised end-to-end.

Impact

• Local server /api/v1/oauth/usage always returns quota.extraUsage: null.
• The booster section in the web settings UI (PlanUsageCard, settings.planUsage.boosterTitle)
  never renders, so users cannot see their booster balance anywhere.

Suggested fix

extraUsage: parseBoosterWallet(payload['booster_wallet'] ?? payload['boosterWallet']),

(reading both keys keeps compatibility if the gateway ever normalizes to camelCase),
and update the test fixture to use the real snake_case wire format, plus add a
booster_wallet entry to the fetchManagedUsage mock response.

Start in packages/oauth/src/managed-usage.ts and read packages/oauth/test/managed-usage.test.ts, especially the parseManagedUsagePayload fixture and fetchManagedUsage mock. Run the managed-usage tests, then verify that the real snake_case booster_wallet response produces quota.extraUsage and that the end-to-end mock covers the booster wallet.