fix(openfeature): treat empty contexts literally (FFL-2837)

[btthomas]

PR stack

1. Plan: #1345
2. Rules engine boundary: #1346
3. Context semantics and precomputed-context helper <- you are here
4. Dynamic rules evaluation: #1347
5. Dynamic offline OpenFeature API: #1348

Summary

This PR makes an empty OpenFeature context a real evaluation context. It does not use an empty context as a request to restore the context in a precomputed configuration.

This PR also exports getPrecomputedContext(configuration). The function returns a detached copy of the context in a valid precomputed branch. It returns undefined when the configuration has no context-specific precomputed branch. The function does not change OpenFeature or provider state.

The provider now preserves the difference between a missing targeting key and an empty targeting key. The online provider behavior does not change.

Reason

OpenFeature gives a provider the effective context. It uses an empty object when the global context is unset or cleared. The provider cannot determine which OpenFeature operation produced that object.

The old implementation treated the empty object as an internal reset command. This behavior did not match the browser SDK. It also made setContext({}) and clearContext() restore private configuration state.

Applications can now use the public helper to set the matching context explicitly before provider registration. Applications do not need to inspect the configuration wire or parsed object.

User impact

• A context-specific precomputed configuration requires a matching effective OpenFeature context.
• setContext({}) uses the empty context.
• clearContext(domain) uses the inherited global context.
• A context-agnostic precomputed configuration still works with any context.
• Context changes do not fetch configuration.

Upstream follow-up

• After openfeature-js-client#353 is released in @datadog/flagging-core, replace the local helper with the upstream re-export.

Validation

• OpenFeature package: 40 tests passed.
• Core flags package: 101 tests passed.
• OpenFeature and core source lint passed.
• OpenFeature package build passed.
• Core package build passed.

The example-app lint command could not start because the current dependency installation does not contain the jest/globals ESLint environment. The changed package source passed lint, and both package builds passed.