fix(Storage): check dtype in at<T> unconditionally (#965)

[yingjerkao]

Summary

Fixes #965.

• Unconditional dtype check in Storage_base::at<T>. Every at<T> specialization wrapped its dtype-mismatch check in if (cytnx::User_debug) (default false), so e.g. Tensor({1}, Type.Float).item<double>() reinterpreted a float buffer as double and returned garbage in normal builds. The checks now run unconditionally, matching data<T>() and back<T>(), which already checked unconditionally. Mismatched at<T>/item<T> calls now throw std::logic_error instead of silently type-punning.
• Review follow-up: the out-of-bound error in the same at<T> specializations formatted cytnx_uint64 index and unsigned long long size through %d (garbage for values ≥ 2³¹, UB in the formatter) — now %llu with explicit casts.

Test fallout fixed (call sites relying on the old silent behavior)

• tests/linalg_test/Arnoldi_Ut_test.cpp: linalg::Max(block).item<double>() on linalg::Eig output (ComplexDouble) — now reads .item().real().
• tests/DenseUniTensor_test.cpp (at test): the const cut tensor was constructed once with default dtype but read as every dtype in the loop — now cloned per-dtype.

Tests

• Storage.AtDtypeMismatchThrows, Tensor.ItemDtypeMismatchThrows — written red-first against the silent-garbage behavior.
• StorageTest.AtDtypeMismatchThrowsForAllDtypes — exhaustive 11×11 dtype/T matrix, iterating the shared TestTools::dtype_list.
• StorageTest.AtOutOfBoundMessageReportsIndexAndSize — pins the bounds-error message content.

Full suite on macOS/AppleClang: 1067 passed; the only failures are the 4 pre-existing linalg_Test.*_return_err_* SVD-truncate failures tracked in #975 (verified identical without this change).

A follow-up branch (in progress) consolidates the 11 copy-pasted at<T>/back<T>/data<T> specializations into one template body via cy_typeid_v<T> so no future specialization can drop the check again.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request removes the cytnx::User_debug guard around type mismatch checks in Storage_base::at<T> specializations, ensuring that type checks are always performed. It also updates out-of-bounds error messages to use %llu with explicit casts to unsigned long long. Corresponding unit tests have been added to verify that type mismatches throw std::logic_error and that out-of-bounds messages report the correct indices. Additionally, a test in Arnoldi_Ut_test.cpp was updated to use .item().real() to avoid type mismatch exceptions. There are no review comments, and we have no further feedback to provide.

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[codecov]

Codecov Report

❌ Patch coverage is 52.27273% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.91%. Comparing base (32079a4) to head (32ad1e8).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/backend/Storage_base.cpp	52.27%	10 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   30.87%   30.91%   +0.04%     
==========================================
  Files         229      229              
  Lines       34757    34759       +2     
  Branches    14409    14398      -11     
==========================================
+ Hits        10730    10745      +15     
+ Misses      16720    16708      -12     
+ Partials     7307     7306       -1     

Flag	Coverage Δ
cpp	30.42% <52.27%> (+0.04%)	⬆️
python	59.41% <ø> (ø)

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Components	Coverage Δ
C++ backend	32.24% <52.27%> (+0.04%)	⬆️
Python bindings	17.28% <ø> (ø)
Python package	59.41% <ø> (ø)

Files with missing lines	Coverage Δ
src/backend/Storage_base.cpp	25.23% <52.27%> (+3.46%)	⬆️

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[yingjerkao]

Heads-up: #979 is stacked on this branch — it refactors the 11 copy-pasted at<T>/back<T>/data<T> specializations in Storage_base.cpp into single template bodies keyed on cy_typeid_v<T>, so the dtype check this PR fixes can't drift per-type again. It should be retargeted to master and merged after this one.