feat: Add isolated API instance functionality

[askpt]

Signed-off-by: André Silva 2493377+askpt@users.noreply.github.com

This PR

This pull request introduces support for isolated API instances in the OpenFeature .NET SDK, enabling the creation of independent API objects that do not share state with the global singleton. This is particularly useful for testing, multi-tenant applications, and dependency injection scenarios. The changes also enforce provider ownership rules to prevent a provider from being registered with more than one API instance at a time, and update documentation and tests to demonstrate and verify this new functionality.

Isolated API Instance Support:

• Added a new static method Api.CreateIsolated() that creates a fully independent Api instance, with its own providers, context, hooks, and event handlers. These instances do not share state with the global singleton or with each other.
• Enforced provider ownership by adding a _boundApiInstance field to FeatureProvider and validating that a provider is not registered with more than one API instance at a time. Attempts to do so will throw an InvalidOperationException. Ownership is cleared when a provider is shut down. [1] [2] [3] [4] [5] [6]

Documentation and Sample Updates:

• Updated samples/Console/README.md and samples/Console/app.cs to demonstrate the creation and use of isolated API instances, including example flag definitions and evaluation. [1] [2]

Testing Enhancements:

• Added a new test TestIsolatedApiAsync() in test/OpenFeature.AotCompatibility/Program.cs to verify the creation, independence, and shutdown of isolated API instances. [1] [2]

Related Issues

Fixes #734

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.51%. Comparing base (98c394f) to head (529bec4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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+ Coverage   93.46%   93.51%   +0.05%     
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  Files          68       68              
  Lines        2952     2975      +23     
  Branches      350      354       +4     
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+ Hits         2759     2782      +23     
  Misses        135      135              
  Partials       58       58              

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[gemini-code-assist]

Code Review

This pull request implements isolated API instances, enabling the creation of independent OpenFeature API instances that maintain their own providers, hooks, and evaluation contexts. The implementation includes a thread-safe mechanism to prevent a single provider from being registered with multiple API instances concurrently. Documentation, usage samples, and extensive unit and AOT compatibility tests have been added. Feedback highlights a potential NullReferenceException in the provider ownership validation if a null provider is passed to the registration methods.