Update test suite with explicit types and hardcoded byte length assertions

[google-labs-jules]

Updated test suite across tests/ and tests-unity/ by replacing var keyword usages with explicit types, adding hardcoded written byte count verifications alongside existing assertions, and verifying GetByteLength on null-only data.

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PR created automatically by Jules for task 4256007964720628582 started by @sator-imaging

[google-labs-jules]

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