✨ Enhance QDMI device library lookup on Unix

[flowerthrower]

Description

While adding tests that use the FoMaC with the DDSIM QDMI device, I found that applications loading MQT Core indirectly still need to set DYLD_LIBRARY_PATH or LD_LIBRARY_PATH, even when the driver and device libraries are installed in the same directory. Our Windows implementation already does this a bit better.

On Unix systems, dlopen does not reliably search the directory of the already-loaded driver library. This PR uses dladdr to locate the loaded QDMI driver and searches its directory when a device library is specified by filename only. Paths that already contain a directory component are still loaded directly. If this sibling lookup fails, the original dlopen behavior is used as a fallback. This actually matches the existing Windows behavior and allows consumers to load MQT-installed QDMI devices without modifying the process-wide library search path.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• I have disclosed the use of AI tools in the PR description as per our AI Usage Guidelines.
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[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-tidy (v22.1.8) reports: 1 concern(s)

• bindings/fomac/fomac.cpp:297:57: warning: [misc-include-cleaner]

  no header providing "size_t" is directly included

     25 |          const QDMI_Program_Format programFormat, const size_t numShots,
        |                                                         ^

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[burgholzer]

Something seems broken in how this PR was set up.

I am also not 100% I get what kind of problems this is trying to solve.
Our CI (in C++ as well as Python) is working just fine and the CI in the IQM repository is also working just fine. (and it is using the DD device)
Am I overlooking something?

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/mqt-core/fomac/FoMaC.hpp	95.6%	1 Missing ⚠️

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