Fix @clavex/react Jest suite (ESM + React interop)

[fcraviolatti]

Problem

@clavex/react's Jest suite does not run cleanly under the monorepo. Build and
type emission (tsup) are unaffected — this is purely a test-harness issue.

Symptoms

1. With the default ts-jest (CommonJS) preset the suite fails to compile:

   src/ClavexProvider.tsx:72:20 - error TS1343: The 'import.meta' meta-property
   is only allowed when the '--module' option is 'es2020', 'es2022', 'esnext',
   'system', 'node16', 'node18', 'node20', or 'nodenext'.
   

   Root cause: the source uses import.meta.env, which requires ESM.

2. Switching to ts-jest/presets/default-esm + useESM: true + module: esnext
   fixes compilation but then breaks React module interop — React.useContext
   resolves to null under jest ESM:

   Cannot read properties of null (reading 'useContext')   (src/context.tsx:45)
   

   5 tests fail because React/context is mis-resolved.

Current workaround

• jest.config.js left on the default ts-jest preset (documented inline).
• CI (.github/workflows/ci.yml) runs npm test with continue-on-error: true
  so the react suite does not block the pipeline.

Goal

Working jest-ESM setup for @clavex/react with correct React CJS/ESM interop,
so the suite passes and continue-on-error can be removed.

Things to try

• ts-jest/presets/default-esm + extensionsToTreatAsEsm + moduleNameMapper
  pinning a single React instance.
• Or migrate the suite to Vitest (native ESM + import.meta, jsdom).