[projmgr] Improve performance of yml file content comparison

[grasci-arm]

Address performance degradation spotted by nightly performance-check:
https://github.com/Open-CMSIS-Pack/cmsis-toolbox/actions/runs/16665044395/job/47169750921

[github-actions]

Test Results

    3 files     21 suites   12m 38s ⏱️
  419 tests   419 ✅ 0 💤 0 ❌
1 257 runs  1 257 ✅ 0 💤 0 ❌

Results for commit e4bc028.

[codecov]

Codecov Report

❌ Patch coverage is 71.87500% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.59%. Comparing base (1c775a0) to head (e4bc028).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/projmgr/test/src/ProjMgrUtilsUnitTests.cpp	28.57%	0 Missing and 5 partials ⚠️
tools/projmgr/src/ProjMgrYamlEmitter.cpp	80.00%	1 Missing and 2 partials ⚠️
tools/projmgr/src/ProjMgrUtils.cpp	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2173      +/-   ##
==========================================
- Coverage   66.59%   66.59%   -0.01%     
==========================================
  Files         181      181              
  Lines       38384    38403      +19     
  Branches    22492    22502      +10     
==========================================
+ Hits        25563    25575      +12     
- Misses       8082     8083       +1     
- Partials     4739     4745       +6     

Flag	Coverage Δ
projmgr-cov	80.77% <71.87%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
tools/projmgr/include/ProjMgrUtils.h	88.88% <ø> (ø)
tools/projmgr/src/ProjMgrUtils.cpp	93.15% <90.00%> (-0.11%)	⬇️
tools/projmgr/src/ProjMgrYamlEmitter.cpp	93.33% <80.00%> (-0.99%)	⬇️
tools/projmgr/test/src/ProjMgrUtilsUnitTests.cpp	42.62% <28.57%> (-0.33%)	⬇️

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