Bump MessageLength to buffer size for supervisor communications

[kuqin12]

Description

The MessageLength has been tied to input buffer size in the recent movement of removing core private data.

This makes the caller having to change to compensate this because the buffer size was meant to indicate the total buffer size the MM core can use for the return.

Given this supervised MM environment has fixed buffer size, already pre-unblocked, it could be treated that the message length will always be covering the entire prepared buffer region.

The test is also reverted to verify the corresponding change.

• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?

How This Was Tested

This was tested on QEMU Q35 and passed the updated test app.

Integration Instructions

N/A

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.52%. Comparing base (cb61dae) to head (12a9ccc).

Files with missing lines	Patch %	Lines
...sorPkg/Drivers/MmPeiLaunchers/Common/MmIplCommon.c	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #650      +/-   ##
========================================
- Coverage   0.52%   0.52%   -0.01%     
========================================
  Files        133     133              
  Lines      20101   20103       +2     
  Branches      60      60              
========================================
  Hits         106     106              
- Misses     19990   19992       +2     
  Partials       5       5              

Flag	Coverage Δ
MmSupervisorPkg	0.52% <0.00%> (-0.01%)	⬇️

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