Clean up PCPATR Preparer a little

[papeh]

• Capture newly-created ICmPossibilityLists directly when created to simplify logic and prevent race condition interceptions (none proven or even likely)
• Remove unused usings
• Make Cache and ShowMessages properties readonly

Quick Summary

There is a problem with the cache not getting updated immediately when these lists are created. I don't expect these changes to fix this problem, but I can't imagine them doing any harm, and they remove a theoretical possibility of interference.

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• Builds/tests pass locally (or I've run the CI-style build via Bash script or MSBuild).
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   11m 2s ⏱️ +9s
5 761 tests ±0  5 680 ✅ ±0  81 💤 ±0  0 ❌ ±0 
5 770 runs  ±0  5 689 ✅ ±0  81 💤 ±0  0 ❌ ±0 

Results for commit dbffd0c. ± Comparison against base commit 82bbefb.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.04%. Comparing base (82bbefb) to head (dbffd0c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1055      +/-   ##
==========================================
+ Coverage   38.01%   38.04%   +0.02%     
==========================================
  Files        1499     1499              
  Lines      349945   350096     +151     
  Branches    40229    40229              
==========================================
+ Hits       133048   133199     +151     
  Misses     187616   187616              
  Partials    29281    29281              

Files with missing lines	Coverage Δ
Src/Utilities/pcpatrflex/PrepFLExDBDll/Preparer.cs	98.08% <100.00%> (+1.78%)	⬆️

... and 4 files with indirect coverage changes

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[AndyBlack]

While this does not fix LT-22649, these are good changes to have. I approve them.