Update CI to use build_type from matrix

[vvish]

Upgrade/fixes for CI and toolchain.

• In gcc and clang CI workflows, ctest switched to use matrix.build_type instead of env.BUILD_TYPE
• In gcc workflow:
  • upgraded the gcc compiler environment from v. 12 to v.14
  • replaced lcov coverage collection with gcovr
  • excluded test/.* and 3p/.* paths from coverage output
  • added an explicit Codecov upload token

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.76%. Comparing base (1e197b6) to head (18cb002).
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Additional details and impacted files

@@             Coverage Diff             @@
##             main      #28       +/-   ##
===========================================
- Coverage   93.46%   81.76%   -11.70%     
===========================================
  Files          15        9        -6     
  Lines         520      181      -339     
  Branches        0       27       +27     
===========================================
- Hits          486      148      -338     
+ Misses         34       12       -22     
- Partials        0       21       +21     

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