Chore: improve concept definitions

[willkill07]

Description

• Clean up concept definitions
• Fix ParallelTests/NodeMultiThread bug

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[dagardner-nv]

The less decltype's the better in my opinion

[willkill07]

/merge

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.16%. Comparing base (10439b3) to head (3565138).
Report is 1 commits behind head on branch-25.02.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-25.02     #521      +/-   ##
================================================
- Coverage         74.18%   74.16%   -0.02%     
================================================
  Files               407      407              
  Lines             15051    15051              
  Branches           1191     1191              
================================================
- Hits              11165    11162       -3     
- Misses             3886     3889       +3     

Flag	Coverage Δ
cpp	70.70% <ø> (-0.01%)	⬇️
py	44.24% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../mrc/include/mrc/coroutines/concepts/awaitable.hpp	0.00% <ø> (ø)

... and 4 files with indirect coverage changes

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