chore: adding predefined sdk wrappers for test data

[joker23]

This PR will create test data wrappers for js-client-sdk and react-sdk to make the tooling more ergonomic.

This PR will also pull in the eslint upgrade.

NOTE: we did not wrap the react-native-sdk test data wrapper because unit testing react native in this way requires a much more robust setup that we are not pursuing yet.

---

Note

Low Risk
Test-only tooling and package exports with no production SDK behavior changes; ESLint upgrade is confined to this package.

Overview
Adds ergonomic offline test helpers for @launchdarkly/js-client-sdk and @launchdarkly/react-sdk via new subpath exports @launchdarkly/client-testing-plugin/js-client-sdk and .../react-sdk.

Each createTestClient builds a client with TestData appended after any user plugins, plus sendEvents: false and streaming: false, and returns { client, testData } so tests can seed initialFlags and mutate flags at runtime. The React entry also exposes createTestClientProvider, which pairs the same client with a pre-wired Provider from createLDReactProviderWithClient.

Build and packaging: tsup now emits the two client modules; optional peer dependencies on the JS and React SDKs were added so consumers import only what they need. Jest coverage verifies seeding, plugin composition, dynamic overrides, and the React Provider helper. Lint tooling in this package was bumped to ESLint 9 with updated scripts.

^{Reviewed by Cursor Bugbot for commit c790a1a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26365 bytes
Compressed size limit: 29000
Uncompressed size: 129044 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38739 bytes
Compressed size limit: 39000
Uncompressed size: 212244 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31979 bytes
Compressed size limit: 34000
Uncompressed size: 114243 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179654 bytes
Compressed size limit: 200000
Uncompressed size: 831422 bytes

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit c790a1a. Configure here.}

[Dobby89]

Please merge this. We can't easily mock LD when using the new @launchdarkly/react-sdk package until this is released.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.