feat: Migrate MultiProvider from js-sdk-contrib

[jonathannorris]

This PR migrates the MultiProvider functionality from the js-sdk-contrib repository into the main js-sdk, providing built-in support for multi-provider evaluation strategies. Addresses #1217

What's Changed

• Added MultiProvider for server SDK with support for multiple evaluation strategies
• Added WebMultiProvider for web SDK with web-specific optimizations
• Included 3 evaluation strategies: FirstMatch, FirstSuccessful, and Comparison
• Comprehensive test coverage for both server and web implementations
• Updated documentation with usage examples and strategy explanations

Testing

• ✅ All existing tests pass
• ✅ New comprehensive test suites for both server and web MultiProvider
• ✅ ESLint compliance across all new code

[gemini-code-assist]

Summary of Changes

Hello @jonathannorris, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates the MultiProvider feature directly into the OpenFeature SDK, enhancing its capability to manage and evaluate feature flags from multiple sources simultaneously. This migration provides a robust framework for scenarios like gradual migrations between flag providers, combining different data sources, or comparing flag values across systems. The implementation includes distinct server and web SDK components, complete with various evaluation strategies and comprehensive testing, streamlining multi-provider operations for developers.

Highlights

• Core Functionality Migration: The MultiProvider functionality has been migrated from js-sdk-contrib directly into the main SDK, providing native support for managing multiple feature flag providers.
• Platform-Specific Implementations: Dedicated MultiProvider implementations are now available for both server-side (MultiProvider) and web-side (WebMultiProvider) SDKs, optimized for their respective environments.
• New Evaluation Strategies: Three distinct evaluation strategies are included: FirstMatchStrategy (default), FirstSuccessfulStrategy, and ComparisonStrategy, offering flexible control over how flags are resolved across multiple providers.
• Enhanced Test Coverage: Comprehensive test coverage has been added for both server and web implementations, ensuring reliability and stability of the new MultiProvider features.
• Updated Documentation: Detailed documentation has been updated for both server and web SDKs, providing clear usage examples and explanations of the new MultiProvider and its strategies.

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[gemini-code-assist]

Code Review

This pull request introduces the MultiProvider functionality for both server and web SDKs, which is a fantastic addition. The implementation is robust and well-tested. I've identified a few areas for improvement, primarily in the documentation to ensure it accurately reflects the implementation, especially for the ComparisonStrategy. Additionally, there are some minor opportunities for code refinement, such as removing redundant logging and a leftover TODO comment. Overall, great work on this significant feature.

[Copilot]

Pull Request Overview

This PR migrates the MultiProvider functionality from the js-sdk-contrib repository into the main OpenFeature JS SDK, providing built-in support for multiple provider evaluation strategies. The implementation includes server and web variants with comprehensive testing and documentation.

Key Changes:

• Added MultiProvider for server SDK and WebMultiProvider for web SDK with support for multiple evaluation strategies
• Implemented three evaluation strategies: FirstMatch, FirstSuccessful, and Comparison
• Added comprehensive test coverage for both server and web implementations
• Updated documentation with usage examples and strategy explanations

Reviewed Changes

Copilot reviewed 30 out of 31 changed files in this pull request and generated 6 comments.

File	Description
packages/server/src/provider/multi-provider/	Server implementation with async evaluation, hook handling, and tracking support
packages/web/src/provider/multi-provider/	Web implementation with synchronous evaluation and web-specific optimizations
packages//test/multi-provider.spec.ts	Comprehensive test suites covering all strategies and edge cases
packages/*/README.md	Updated documentation with usage examples and Multi-Provider sections

Comments suppressed due to low confidence (1)

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