add unit tests for scan/key parser helpers

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Add unit tests for local parser helpers in nx-sdk DB module (parse_scan_rows, parse_keys, and related decoding logic).

Why

• parser helpers are pure logic and ideal for low-risk tests
• malformed payload handling is important for SDK robustness
• this improves confidence without touching host/runtime behavior

Scope

• add tests in crates/nx-sdk/src/db.rs (or adjacent module test block)
• cover valid payloads and malformed/truncated payloads
• verify expected NxError behavior

Done when

• cargo test -p nx-sdk passes with new parser tests
• both happy-path and failure-path cases are covered
• no public API changes are required

Notes

Please keep test data small and readable.

Thanks again for considering contributing to Numax.