Upgrade eslint-config-expensify to 4.0.6

[roryabraham]

Summary

• Upgrades eslint-config-expensify from 2.0.108 to 4.0.6 and replaces the monolithic import expensify from 'eslint-config-expensify' config with modular sub-configs (browser, react, typescript, jest, scripts), matching the pattern used in expensify-common.
• Removes the hand-rolled @typescript-eslint plugin/parser block and ~40 per-rule overrides that duplicated what v4 now ships (type imports/exports, import extensions, redeclare handling, JSDoc disables, etc.).
• Adds eslint-seatbelt with an initial baseline (eslint.seatbelt.tsv) to grandfather existing type-checked violations from the jump to recommendedTypeChecked, without blocking CI.
• Removes unused direct ESLint plugin dependencies (eslint-plugin-import, eslint-plugin-jsx-a11y, eslint-plugin-react, @typescript-eslint/*, @lwc/eslint-plugin-lwc, @react-native-community/eslint-config) now provided transitively by eslint-config-expensify.
• Removes stale eslint-disable comments for rules that v4 no longer reports (dot-notation, @typescript-eslint/no-empty-function).

Overrides retained (by category)

Onyx / project-structure specific (intentional):

Override	Scope	Why
rulesdir/no-multiple-onyx-in-file: off	all source	This repo is Onyx; tests and mocks intentionally create multiple instances
import/extensions: off	tests/**, mocks	Test utilities import without extensions
no-restricted-syntax (allow for...of / for...in)	tests	Tests use iteration patterns blocked by the default airbnb syntax rule
@typescript-eslint/no-require-imports: off	**/*.native.ts(x)	React Native platform files use require()
globals.jest	MemoryOnlyProvider.ts	Test-only provider accessed from Jest mocks
globalIgnores for .github/**, dist/**, cpp/**, etc.	n/a	Bundled GH Action output, build artifacts, and local bench/C++ dirs are not lint targets

Not Onyx-specific (generic library / TypeScript conventions — candidates to remove over time):

Override	Why it exists
class-methods-use-this: off	Same relaxation App uses for TS/React codebases
es/no-optional-chaining: off	Expensify-wide relaxation (Hermes / toolchain compatibility)
es/no-nullish-coalescing-operators: off	Expensify-wide relaxation
react/prop-types: off	TypeScript project; prop-types are redundant
react/require-default-props: off	TypeScript project; default props enforced via types
react/jsx-props-no-spreading: off	Library hook/components (useOnyx) spread props intentionally

Deliberately not imported:

• eslint-config-expensify/expensify — contains App-only rulesdir/* rules (prefer-onyx-connect-in-libs, no-onyx-connect, restricted react-native imports, etc.) that do not apply to this package.

Follow-up work

• Gradually burn down eslint.seatbelt.tsv by fixing type-checked violations (813 grandfathered warnings), especially @typescript-eslint/no-unsafe-* in generic Onyx typing code and tests.
• Re-evaluate whether the six generic overrides above can be dropped once violations are fixed.

Test plan

1. Run npm ci and confirm dependencies install cleanly.
2. Run CI=true npm run lint and confirm it exits 0 (seatbelt frozen mode).
3. Run npm run build and confirm TypeScript compilation succeeds.
4. Run npm test and confirm the unit test suite passes.

[roryabraham]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve undefined updates instead of writing empty objects

When Onyx.update() batches a SET/MULTI_SET entry whose value is undefined (for example from a JS caller or an optional Partial multiSet property), the queue is [null, undefined] and Onyx.ts calls mergeChanges(operations) before forwarding the result to set(). This new fallback turns that undefined into {} because no existingValue is supplied, so an update that should be ignored like Onyx.set()/multiSet() currently do will instead write an empty object and notify subscribers. Keep the old undefined result here or skip undefined set entries before batching.

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