Fix #153, use sizeof for snprintf buffer sizes in DS_SendHkCmd

[chillfig]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution

• Fixes DS_SendHkCmd snprintf buffer size arguments mismatch #153, sets the buffer allocation to match the actual size of the destination buffer

Testing performed
build, tests, lcov

Expected behavior changes
Nothing functional. Ensures write limit is derived directly from the destination buffer's declared size. Prevents overflow of truncation in the event of a user changing the configuration.

System(s) tested on

• Hardware: aarch64
• OS: Ubuntu 24.04.4 LTS
• Versions 0cd8596 commit of apps/ds + 38eaf4244185bc70aa296f1d8df7687e2f13989d cFS bundle

Additional context
N/A

Third party code
N/A

Contributor Info - All information REQUIRED for consideration of pull request
Justin Figueroa, Vantage Systems Inc