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.

[gemini-code-assist]

While checking result.ok() is necessary to handle explicit provider errors, it is also important to ensure that the provider did not return a nullptr inside the absl::StatusOr. If a provider implementation returns absl::OkStatus() but a null unique_ptr, the subsequent dereference in the caller (e.g., GetBooleanValue) will cause a crash. Given the PR's objective to prevent abnormal termination, adding a null check here provides an essential layer of safety.

  if (!result.ok() || *result == nullptr) {
    return std::make_unique<ResolutionDetailsType>(
        default_value, Reason::kError, std::nullopt, FlagMetadata(),
        ErrorCode::kGeneral,
        result.ok() ? "Provider returned null resolution details"
                    : std::string(result.status().message()));
  }
  return std::move(*result);