[Tests] Fix undefined behavior in shammath/solve::least_squares test

[tdavidcl]

The comparison loop used an uninitialized loop counter (size_t i; instead of size_t i = 0;), so its starting value was whatever garbage was left on the stack. Depending on compiler/optimization level, this could skip comparisons entirely (silently passing a bad fit) or read out of bounds, explaining the flaky/non-deterministic CI failures. Also fixed a related out-of-bounds read: ex_res only had 3 entries but was indexed up to 3 (missing the expected R^2 value).

Assisted-by: Claude

[github-actions]

Thanks @tdavidcl for opening this PR!

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Reviewing files that changed from the base of the PR and between 92cccdf and 6b53feb.

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• src/tests/shammath/matrixTests.cpp

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[github-actions]

Workflow report

workflow report corresponding to commit 6b53feb
Commiter email is noreply@anthropic.com

Light CI is enabled (the default for pull requests). This will only run the basic tests and not the full tests.
Full CI runs if the full-ci label is set, or automatically on Mergify merge-queue branches (mergify/merge-queue/*).
The merge gate job "on PR / all" is skipped in this case. Queue entry uses "on PR / all_light"; full CI runs in the merge queue.

Pre-commit check report

Pre-commit check: ✅

trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check for merge conflicts................................................Passed
check that executables have shebangs.....................................Passed
check that scripts with shebangs are executable..........................Passed
check for added large files..............................................Passed
check for case conflicts.................................................Passed
check for broken symlinks................................................Passed
check yaml...............................................................Passed
detect private key.......................................................Passed
No-tabs checker..........................................................Passed
Tabs remover.............................................................Passed
cmake-format.............................................................Passed
Validate GitHub Workflows................................................Passed
clang-format.............................................................Passed
ruff check...............................................................Passed
ruff format..............................................................Passed
Check doxygen headers....................................................Passed
Check license headers....................................................Passed
Check #pragma once.......................................................Passed
Check SYCL #include......................................................Passed
No ssh in git submodules remote..........................................Passed
No UTF-8 in files (except for authors)...................................Passed

Test pipeline can run.

Clang-tidy diff report

No relevant changes found.
Well done!

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Doxygen diff with main

Removed warnings : 0
New warnings : 0
Warnings count : 8183 → 8183 (0.0%)

Detailed changes :