fix: Correct logic for evaluation blocked for ERROR and STALE providers

[NeaguGeorgiana23]

This PR

Fixes the flag evaluation which was incorrectly blocked when the feature provider was in ERROR or STALE states.

Previously, the SDK short-circuited evaluation for any provider status other than READY. This PR updates the EvaluateFlag logic to strictly follow the OpenFeature specification by:

• Short-circuiting and returning the PROVIDER_NOT_READY error code only when the provider is NOT_READY (Requirement 1.7.6).
• Short-circuiting and returning the PROVIDER_FATAL error code only when the provider is in a FATAL error state (Requirement 1.7.7).
• Allowing evaluation to proceed for READY, ERROR, and STALE states, ensuring cached values can still be returned when appropriate.

Related Issues

Fixes #69

Notes

• Implements compliance for OpenFeature specification Requirements 1.7.6 and 1.7.7.
• Remaining minor bugs from the tracking issue will be addressed in subsequent PRs.

Follow-up Tasks

• Address remaining minor bugs in the compliance tracking issue

[gemini-code-assist]

Code Review

This pull request updates the EvaluateFlag method in ClientAPI to explicitly handle different provider statuses. It replaces the generic non-ready check with specific logic for ProviderStatus::kNotReady and ProviderStatus::kFatal, ensuring that a fatal provider state returns the appropriate ErrorCode::kProviderFatal. I have no feedback to provide as there were no review comments to evaluate.