ci: test secure-memory builds in the CMake matrix

[Alb3e3]

Fixes #1712.

Add secure-memory ON/OFF builds to the existing CMake matrix across Ubuntu,
Windows, macOS and C++11/17/20. Use Debug consistently across generators and run
CTest after building, including the reader/writer suites as well as unit tests.

Verified locally with GCC 15.2: all three CTest suites pass in each of the six
secure-memory/C++ configurations. The pre-#1709 secure build fails on string
allocator mismatches. Restoring only the old CZString cleanup call on current
master also makes the secure CTest run fail at ValueTest/CZStringCoverage.
Actionlint passes. Windows and macOS still need the GitHub runners.

Prepared with AI assistance; the validation commands above were run locally.

[greptile-apps]

Greptile Summary

The PR expands the CMake CI matrix to exercise secure-memory enabled and disabled builds across three operating systems and C++11/17/20. It also standardizes Debug builds and runs the registered CTest suites after each build.

• Adds JSONCPP_USE_SECURE_MEMORY ON/OFF matrix coverage.
• Uses a consistent build directory and Debug configuration.
• Runs CTest with failures and missing tests treated as CI failures.

Confidence Score: 5/5

The PR appears safe to merge, with the new matrix and test invocation using consistent CMake configuration and build paths across supported runners.

The workflow correctly passes secure-memory and language-standard values as CMake definitions, handles single- and multi-config Debug builds, and runs tests from the configured build directory.

Important Files Changed

Filename	Overview
.github/workflows/cmake.yml	Expands the existing matrix to 18 secure-memory/platform/language configurations and adds consistent Debug build and CTest execution; no actionable defect was identified.

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