Add CI test for relative path handling in STM32_CUBE_<FAMILY>_PATH

[Copilot]

Relative paths specified via STM32_CUBE_<FAMILY>_PATH fail during CMSIS discovery. This adds a CI test to validate the behavior.

Changes

• Added relative path test to .github/workflows/cmake.yml
  • Configures tests/cmsis with relative path to fetched STM32Cube packages
  • Path: ../../fetch/build/_deps/stm32cube<family>-src from build directory
  • Validates FindCMSIS.cmake can locate CMSIS headers/sources via relative paths
  • Runs for all families except MP1 (consistent with existing tests)

Test Flow

# Working directory: /path/to/tests/cmsis/build-relative
cmake -DSTM32_CUBE_F4_PATH=../../fetch/build/_deps/stm32cubef4-src \
      -DTEST_FAMILIES=F4 \
      /path/to/tests/cmsis/

The test currently exposes the relative path bug - it will fail until FindCMSIS.cmake properly resolves relative paths in find_path() calls.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Relative paths fail consistently</issue_title>
<issue_description>Describe the bug
If your Cube directories are not in the default paths and you use relative paths for STM32_CUBE_<FAMILY>_PATH it will fail to find CMSIS.

Sources to reproduce
Please provide link to repository showing the problem.

• In main branch

Steps to reproduce

1. Download of fetch Cube code from examples/fetch-cube
2. Place it somewhere other than the default location of /opt/STM32Cube<FAMILY>
3. Go to root/examples/blinky
4. Create a build folder and cd build
5. Run cmake .. -DSTM32_CUBE_F4_PATH=<absolute-path>/stm32-cmake/thirdParty/STM32CubeF4
6. This builds successfully.
7. rm -rf ./* in build
8. Run cmake .. -DSTM32_CUBE_F4_PATH=<relative-path>/stm32-cmake/thirdParty/STM32CubeF4 leads to
   CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:290 (message): Could NOT find CMSIS (missing: CMSIS_INCLUDE_DIRS CMSIS_SOURCES STM32F4)

Expected behavior
Expected behavior is that this project should be able to handle relative paths or at least mention it in big bold letters in the documentation.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: Linux x86_64
• Compiler: arm-none-eabi-gcc 15.2.Rel1
• stm32-cmake: ecc5acc
• cmake: 4.2.2

Additional context
Add any other context about the problem here.
</issue_description>

<agent_instructions>Add a test in .GitHub/workflows to be ran by the CI using tests/cmsis folder.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Relative paths fail consistently #363

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