Validate cuOptGetErrorString buffer size

[fallintoplace]

This tightens one C API edge case and adds a small regression test for it.

What changed:

• reject non-positive buffer sizes in cuOptGetErrorString
• add a focused C API test that checks 0 and -1 buffer sizes

Why:
cuOptGetParameter already rejects <= 0 output buffer sizes, but cuOptGetErrorString was still passing its signed size straight into snprintf. This just makes the behavior consistent and keeps the validation on the API boundary.

Testing:

• git diff --check
• not run here: this machine does not have nvcc or a configured cuOpt build environment, so I could not build libcuopt / run C_API_TEST locally

Closes #1311.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between d415bb6 and bd69111.

📒 Files selected for processing (4)

• cpp/src/pdlp/cuopt_c.cpp
• cpp/tests/linear_programming/c_api_tests/c_api_test.c
• cpp/tests/linear_programming/c_api_tests/c_api_tests.cpp
• cpp/tests/linear_programming/c_api_tests/c_api_tests.h

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📝 Walkthrough

Walkthrough

This PR adds input validation to the cuOptGetErrorString C API function to reject non-positive buffer sizes. The implementation guard returns CUOPT_INVALID_ARGUMENT when error_string_size <= 0, matching the existing validation pattern in cuOptGetParameter. A new test creates an optimization problem, solves it, and verifies the function correctly rejects zero and negative buffer sizes before cleanup.

Changes

Error String Buffer Size Validation

Layer / File(s)	Summary
Implementation: buffer size validation in cuOptGetErrorString cpp/src/pdlp/cuopt_c.cpp	Added early return guard in cuOptGetErrorString to reject error_string_size <= 0 before passing to std::snprintf, returning CUOPT_INVALID_ARGUMENT.
Test coverage for invalid buffer sizes cpp/tests/linear_programming/c_api_tests/c_api_tests.h, cpp/tests/linear_programming/c_api_tests/c_api_test.c, cpp/tests/linear_programming/c_api_tests/c_api_tests.cpp	Added test function declaration, C test implementation that constructs a problem, calls the API with zero and negative buffer sizes, verifies rejection, and GoogleTest wrapper to integrate with the test suite.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Validate cuOptGetErrorString buffer size' clearly and concisely describes the main change: adding validation for buffer size in the cuOptGetErrorString function.
Description check	✅ Passed	The description explains the change (rejecting non-positive buffer sizes), the reasoning (consistency with cuOptGetParameter), and notes testing limitations, all directly related to the changeset.
Linked Issues check	✅ Passed	The PR fully addresses issue #1311 by implementing the validation check in cuOptGetErrorString and adding a regression test for buffer sizes 0 and -1.
Out of Scope Changes check	✅ Passed	All changes are directly related to the linked issue requirements: validation in cuOptGetErrorString and tests for the new behavior. No out-of-scope changes detected.

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