fix: provider exceptions crash the caller

[NeaguGeorgiana23]

This PR

Resolves the issue where feature provider errors could propagate and crash the caller during flag evaluation.

To satisfy Requirement 1.4.10 while strictly adhering to the Google C++ Style Guide's zero-exception rules, this PR refactors the evaluation boundary to use compile-time contractual safety:

• Updates the FeatureProvider evaluation interface to return absl::StatusOr.
• Updates the client evaluation logic to gracefully handle failure statuses. If a provider returns an error status, the SDK safely intercepts it and returns the default value populated with a general error code.
• Prevents abnormal termination

Related Issues

Fixes #69

Follow-up Tasks

• Address remaining minor bugs from the Specification Compliance tracking issue.

[gemini-code-assist]

Code Review

This pull request updates the FeatureProvider interface and its implementations, including InMemoryProvider and NoopProvider, to return absl::StatusOr<std::unique_ptr<...>> for all flag evaluation methods. This change standardizes error reporting and explicitly discourages the use of C++ exceptions. Corresponding updates were made to the ClientAPI, test suites, and mocks to handle the new return types. Feedback suggests adding a null check for the unique_ptr within the absl::StatusOr in openfeature/client_api.h to prevent potential crashes if a provider returns an OK status with a null pointer.

[toddbaert]

Mostly LGTM. A few things:

• No tests for the 3 new branches in EvaluateFlag; cheap to add with MockFeatureProvider + a small throwing mock.
• Gemini's null-check point seems right
• I think this breaks the FeatureProvider (correct me if I'm wrong, I'm not that strong at C++). That's totally fine since we are not 1.0, but we should use a fix!: prefix.

I'll approve now, up to you to make these changes if you agree. ❤️