feat: add Provider Pattern API and modernize test suite

[luishcastroc]

Angular SDK plugin

What did you accomplish?

Provider Pattern API (New Feature)

• Add provideSplitIo() function with withConfig() and withFeatureOptions() for Angular standalone API
• Add SplitIoService with automatic initialization and dependency injection
• Add functional guards: splitIoReadyGuard, splitIoReadyMatchGuard, createTreatmentGuard(), createMultiTreatmentGuard()
• Remove redundant splitIoReadyChildGuard (use splitIoReadyGuard for both canActivate and canActivateChild)

Signal Support (New Feature)

• Add signal-based reactive state management alongside Observables for Angular 16+ compatibility
• Add isReadySignal, isReadyFromCacheSignal, isTimedOutSignal, and hasUpdateSignal properties
• Signals synchronized with existing Observable state for dual API support
• Enable use with computed() and effect() for reactive programming patterns
• Maintain backward compatibility with Observable-based patterns

Test Modernization

• Convert all 22 tests from done() callbacks to async/await with firstValueFrom
• Add 9 comprehensive tests for signal functionality
• Eliminate nested subscription anti-patterns in complex Observable tests
• Fix test patterns for multiple Observable emissions using proper async/await sequencing

Type Safety Improvements

• Improve guard typing: use SplitIO.Treatment instead of generic string
• Use SplitIO.Attributes instead of any for attributes parameter
• Better type safety across guard functions

Documentation Updates

• Add comprehensive signal examples to README.md with computed() and effect() usage
• Add signal migration guide section to MIGRATION.md
• Standardize terminology to "Provider Pattern" throughout documentation
• Update CONTRIBUTORS-GUIDE.md to reflect main branch workflow (removed outdated development references)

How do we test the changes introduced in this PR?

npm test

All 81 tests passing with 90.2% line coverage, 84.88% branch coverage maintained.

Key test scenarios:

• Provider pattern initialization with provideSplitIo()
• Functional guards with async/await patterns
• Service methods: getTreatment(), createClient(), track(), etc.
• SDK event handling (ready, timeout, update events)
• Multi-client support with shared keys
• Error handling for uninitialized clients
• Signal state synchronization with Observables
• Signal readonly properties and reactive contexts

Extra Notes

• This maintains backward compatibility - the legacy SplitService and SplitioGuard are still available but marked as deprecated
• The new Provider Pattern follows Angular 14+ standalone API conventions
• Signal support provides a path forward for Angular 16+ applications while keeping Observable support for existing codebases
• All async test patterns now use async/await instead of callbacks, making tests easier to read and maintain
• Type safety improvements help catch errors at compile time rather than runtime
• Signals offer better performance for synchronous state updates and simpler syntax with computed values