🧪 Add varint_decode_i16 tests

[panayang]

🎯 What: Added tests for varint_decode_i16 in decode_signed.rs
📊 Coverage: Covered boundary conditions (positive max, negative max, etc.) as well as zero and negative/positive normal numbers. Covered error conditions like unexpected end and invalid integer type decodings. Included both big and little endian modes.
✨ Result: Improved test coverage for signed integer decoding.

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PR created automatically by Jules for task 6548754917195442794 started by @panayang

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