Changed UpdateFlags function to return absl::Status

[m-olko]

This PR

Update the FlagSync:UpdateFlags interface to return absl::Status and return errors on validation problems. The change also includes updating all the tests to support that.

Related Issues

Fixes #113

[gemini-code-assist]

Code Review

This pull request refactors the flag synchronization and validation logic in the flagd provider to return absl::Status instead of boolean or void, allowing for better error propagation. Callers and test suites have been updated to handle and assert on these statuses. The reviewer's feedback focuses on performance and safety improvements, suggesting the use of [[nodiscard]] to prevent ignoring returned statuses, passing JSON objects by value to enable move semantics and avoid expensive copies, and using auto for clearly deduced types.

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