Fix nasa/cFS#703, Reduce stack usage in UT_CallTaskPipe

[heathdutton]

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
Moves the large message buffer in UT_CallTaskPipe() from stack to static storage. The buffer is CFE_MISSION_SB_MAX_SB_MSG_SIZE bytes (32KB by default), which exceeds the default RTEMS task stack size of 20KB configured in bsp_start.c.

Fixes nasa/cFS#703

Testing performed
Code review and verification that the change follows existing patterns in the file (other static buffers like UT_CFE_ES_ResetData).

Expected behavior changes
Unit tests will no longer overflow the stack on systems with constrained stack sizes (e.g., RTEMS with default 20KB stacks).

System(s) tested on
Code review only (macOS development environment does not support cFS build)

Contributor Info - All information REQUIRED for consideration of pull request
Heath Dutton