feat: Extract minimal DI integration to OpenFeature.DependencyInjection.Abstractions

[arttonoyan]

This PR

Extract the minimal, provider-agnostic DI surface into a new package: OpenFeature.DependencyInjection.Abstractions.
This isolates the contracts and lightweight wiring needed to integrate any OpenFeature provider without pulling in a concrete implementation.

Related Issues

Fixes #587

Notes

• Options split: Extracted provider-agnostic configuration from OpenFeatureOptions into a new base options type: OpenFeatureProviderOptions.
• Inheritance: OpenFeatureOptions now inherits from OpenFeatureProviderOptions.
• Internal wiring update: Internal configuration that previously targeted OpenFeatureOptions now targets OpenFeatureProviderOptions.

Before (internal)

services.AddOptions<OpenFeatureOptions>()
    .Configure(options =>
    {
        options.AddProviderName(null);
    });

After (internal)

services.AddOptions<OpenFeatureProviderOptions>()
    .Configure(options =>
    {
        options.AddProviderName(null);
    });

Note: This is an internal refactor. It does not affect consumers unless they were directly modifying OpenFeatureOptions via DI options configuration (which is not a supported/typical usage).

---

Impact

• No new functionality.
• Behavior unchanged at runtime (same provider selection, same evaluation outcomes, same logs).
• Binary/source compatibility: Consumers using the documented extension methods and standard registration remain unaffected.
• Potential touchpoints: Only internal code paths (or unconventional consumer code directly configuring OpenFeatureOptions) require updates to target OpenFeatureProviderOptions.

---

How to test

This is a regression-only refactor.

Expectations

• All existing unit/integration tests pass unchanged.
• No DI resolution or startup configuration differences.

[codecov]

Codecov Report

❌ Patch coverage is 96.85039% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.44%. Comparing base (d15960c) to head (acade51).

Files with missing lines	Patch %	Lines
...stractions/OpenFeatureProviderBuilderExtensions.cs	90.24%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
- Coverage   89.52%   89.44%   -0.09%     
==========================================
  Files          77       80       +3     
  Lines        3084     3097      +13     
  Branches      350      355       +5     
==========================================
+ Hits         2761     2770       +9     
  Misses        256      256              
- Partials       67       71       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.