Solve message requirements casting issue

[nesitor]

Fix: Solve message class casting issue with requirements field.

Self proofreading checklist

• The new code clear, easy to read and well commented.
• New code does not duplicate the functions of builtin or popular libraries.
• An LLM was used to review the new code and look for simplifications.
• New classes and functions contain docstrings explaining what they provide.
• All new code is covered by relevant tests.
• Documentation has been updated regarding these changes.
• Dependencies update in the project.toml have been mirrored in the Debian package build script packaging/Makefile

Changes

Removed InstanceContent casting and put it as ExecutableContent.

How to test

Create a VM with GPU and check that the VM is running.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.56%. Comparing base (28582cf) to head (e7570cd).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/pool.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #800      +/-   ##
==========================================
+ Coverage   64.53%   64.56%   +0.02%     
==========================================
  Files          78       78              
  Lines        7092     7092              
  Branches      598      598              
==========================================
+ Hits         4577     4579       +2     
+ Misses       2315     2313       -2     
  Partials      200      200              

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