Fix block Svd_truncate and Gesvd_truncate return_err output

[yingjerkao]

Fixes #762.

Summary

• make block Svd_truncate and Gesvd_truncate return discarded singular values in the return_err path
• make return_err == 1 return the first discarded singular value instead of the smallest kept value
• keep block output ordering consistent with the dense truncation path
• add regressions for both block APIs and the return_err == 1 edge case

Verification

• ./build/openblas-cpu/tests/test_main --gtest_filter="linalg_Test.BkUt_Svd_truncate_return_err_returns_discarded_values:linalg_Test.BkUt_Gesvd_truncate_return_err_returns_discarded_values:linalg_Test.BkUt_Svd_truncate_return_err_one_returns_first_discarded_value:linalg_Test.BkUt_Gesvd_truncate_return_err_one_returns_first_discarded_value"
• ./build/openblas-cpu/tests/test_main --gtest_filter="Svd_truncate.*:Gesvd_truncate.*:linalg_Test.BkUt_Svd_truncate1:linalg_Test.BkUt_Svd_truncate2:DenseUt_Svd_truncate:DenseUt_Gesvd_truncate"

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.58%. Comparing base (a22dd1e) to head (1ede487).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linalg/Gesvd_truncate.cpp	75.00%	3 Missing and 1 partial ⚠️
src/linalg/Svd_truncate.cpp	75.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
+ Coverage   35.47%   35.58%   +0.11%     
==========================================
  Files         214      214              
  Lines       33007    33013       +6     
  Branches    13130    13145      +15     
==========================================
+ Hits        11709    11748      +39     
+ Misses      19376    19350      -26     
+ Partials     1922     1915       -7     

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