feat(build): add --clean-cache option to case.build

[mahf708]

Reset only the CMake cache (CMakeCache.txt and the small bookkeeping files in CMakeFiles/) without touching compiled object files. This is useful when CMake needs to re-configure (e.g. after editing macros or toolchain settings) but a full --clean-all is too expensive.

Adds:

• --clean-cache flag in CIME/Tools/case.build (mutually exclusive with the existing --clean-* options)
• build._clean_cache_impl and a clean_cache=True path through build.clean, logged under a distinct "build.clean_cache" phase
• pytest unit tests covering the happy path, no-op when no cmake-bld dir exists, missing CMakeCache.txt, and dispatch routing in build.clean

Description

• Closes #<ISSUE_NUMBER_HERE>

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.41%. Comparing base (c19d517) to head (743e8f7).
⚠️ Report is 53 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4978      +/-   ##
==========================================
+ Coverage   28.29%   28.41%   +0.11%     
==========================================
  Files         262      262              
  Lines       38390    38446      +56     
  Branches     8126     8138      +12     
==========================================
+ Hits        10863    10924      +61     
+ Misses      26279    26261      -18     
- Partials     1248     1261      +13     

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[jasonb5]

LGTM