[csolution-rpc] Clean active target set before loading a solution

[brondani]

Fixes

• Active target-set selection unexpectedly persists after opening a different solution vscode-cmsis-solution#302

Changes

• Clean active target set before loading a solution, ensuring a previously loaded target set (and its project-context selection) is not inadvertently reused.
• Add test case.

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.24%. Comparing base (7b35e61) to head (0bcd081).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2496   +/-   ##
=======================================
  Coverage   65.24%   65.24%           
=======================================
  Files         147      147           
  Lines       26674    26675    +1     
  Branches    16166    16166           
=======================================
+ Hits        17404    17405    +1     
  Misses       7071     7071           
  Partials     2199     2199           

Flag	Coverage Δ
projmgr-cov	87.93% <100.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
tools/projmgr/include/ProjMgrWorker.h	83.87% <100.00%> (+0.53%)	⬆️

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[github-actions]

Test Results

    3 files     21 suites   20m 36s ⏱️
  467 tests   467 ✅ 0 💤 0 ❌
1 401 runs  1 401 ✅ 0 💤 0 ❌

Results for commit 0bcd081.