ci(contract-test-utils): consolidating test websocket implementation

[joker23]

Note

Medium Risk
Moderate risk because it refactors shared contract-test infrastructure (WebSocket command routing and SDK config construction) used by multiple entity implementations, so regressions could break test harness interactions across platforms.

Overview
Consolidates client-side contract test entity plumbing into @launchdarkly/js-contract-test-utils by adding shared TestHarnessWebSocketBuilder, ClientEntity, doCommand dispatch, and a reusable ConfigBuilder for translating harness config into SDK options.

Updates the browser, React, and React Native contract-test entities to use the shared builder/entity/config utilities, removing their bespoke WebSocket servers and per-SDK command/config duplication while preserving existing capability declarations and initialization semantics (timeouts/initCanFail).

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25661 bytes
Compressed size limit: 29000
Uncompressed size: 126143 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179331 bytes
Compressed size limit: 200000
Uncompressed size: 829227 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 37015 bytes
Compressed size limit: 38000
Uncompressed size: 202741 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31556 bytes
Compressed size limit: 34000
Uncompressed size: 112044 bytes

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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[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.

[joker23]

moving this back to draft because the test adapter consolidation is more impactful and would likely change what we are doing here