Enforce Core's ESLint rules on perps source files so the codebase is
ready to move into @metamask/perps-controller without a wall of lint
errors at extraction time.

Key changes:
- Add perps-specific ESLint override matching @metamask/eslint-config
- Convert `in` operator usage to Object.hasOwn()
- Replace || with ?? where nullish coalescing is more correct
- Use ES private class fields (#field) instead of TS private keyword
- Enforce import ordering, explicit return types, JSDoc standards
- Test files excluded from strict enforcement via excludedFiles

…ing-core-steps

The previous ESLint alignment commit (21b0d13) introduced 59 TS errors.
This fixes all of them by reverting changes that conflict with mobile's
tsconfig (lib: es2017) and established patterns:

- Revert Object.hasOwn() → 'in' operator (28 TS2550 + 5 TS2339 errors)
  Object.hasOwn() requires lib:es2022, and unlike `in`, doesn't provide
  TS type narrowing. Disabled the rule in .eslintrc.js for mobile.

- Revert !== null → != null for optional chaining (14 TS18048 errors)
  The != null pattern guards both null and undefined with ?. chains.
  Disabled no-eq-null in .eslintrc.js for mobile.

- Fix num → number variable references (3 TS2304 errors)
  ESLint id-denylist renamed vars but missed some references.

- Fix ?? → || for truthy checks on object refs (1 error)
- Fix isNaN() with possibly-undefined args (4 TS2345/TS18048 errors)
- Fix redundant condition in MYXClientService (1 TS2774 error)
- Fix test types: GetAvailableDexsParams, sl_price→slPrice, private
  field access (6 test TS errors)

Both deferred rules are tracked for enforcement in Core where lib will
be es2022+ and the patterns can be cleanly adopted.

- Revert `current === null` back to `current == null` in
  setSelectedPaymentToken — the state field can be `undefined`
  (fresh state, migration), and `== null` correctly catches both
  null and undefined for the 'perps_balance' analytics fallback.

- Fix sl_price/tp_price → slPrice/tpPrice in PerpsController.test.ts
  and TradingService.test.ts — the method signature was renamed to
  camelCase but test assertions still used snake_case, silently passing
  due to `any` typing on the test helper.

…ing-core-steps

# Conflicts:
#	app/controllers/perps/PerpsController.ts
#	app/controllers/perps/providers/HyperLiquidProvider.ts
#	app/controllers/perps/services/HyperLiquidSubscriptionService.ts
#	app/controllers/perps/utils/standaloneInfoClient.ts

… rule

- Add missing AccountTreeControllerSelectedAccountGroupChangeEvent import
- Rename SCREAMING_SNAKE_CASE class properties to camelCase
  (#PRELOAD_WATCHED_PATHS, #PRELOAD_REFRESH_MS, #PRELOAD_GUARD_MS)
- Replace `void` fire-and-forget with `.catch()` (Core bans no-void)
- Expand @typescript-eslint/naming-convention from 2 to 10+ selectors
  to match Core's @metamask/eslint-config-typescript
- Add `no-void: error` to perps ESLint override

…er pattern

The Record<string, never> type is a forward-compatibility placeholder
so the API signature stays stable when filters/pagination are added
later. Add a comment to prevent future confusion.