🏁 Fix dynamic loading of QDMI device DLLs on Windows when an absolute path is provided (backport #1720) (#1722)

## Description

This fixes an oversight in #1694 that broke dynamic QDMI library loading
on Windows and was discovered in
iqm-finland/QDMI-on-IQM#63
If an absolute path is provided, the implementation now falls back to
the previously-working one that used the simpler `LoadLibraryW` for
loading the DLL.

I'll to a test run of this PR in the QDMI-on-IQM repo before we merge
and release.

AI (Claude Code) was used to diagnose the issue and come up with
potential solutions.
The actual solution was implemented manually.

## Checklist



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

**If PR contains AI-assisted content:**

- [x] I have disclosed the use of AI tools in the PR description as per
our [AI Usage
Guidelines](https://github.com/munich-quantum-toolkit/core/blob/main/docs/ai_usage.md).
- [x] AI-assisted commits include an `Assisted-by: [Model Name] via
[Tool Name]` footer.
- [x] I confirm that I have personally reviewed and understood all
AI-generated content, and accept full responsibility for it.



(cherry picked from commit 09bd043)

Co-authored-by: Lukas Burgholzer <burgholzer@me.com>

Author: mergify[bot]

CHANGELOG.md

@@ -13,6 +13,10 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 - 🚸 Improve native gate support for the Qiskit-to-OpenQASM3 conversion in the QDMI-Qiskit interface ([#1719]) ([**@burgholzer**])
 
+### Fixed
+
+- 🏁 Fix dynamic loading of QDMI device DLLs on Windows when an absolute path is provided ([#1720]) ([**@burgholzer**])
+
 ## [3.6.0] - 2026-05-13
 
 _If you are upgrading: please see [`UPGRADING.md`](UPGRADING.md#360)._
@@ -370,8 +374,9 @@ _📚 Refer to the [GitHub Release Notes](https://github.com/munich-quantum-tool
 [3.0.0]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.0.0
 [2.7.0]: https://github.com/munich-quantum-toolkit/core/releases/tag/v2.7.0
 
-<!-- PR links -->
+<!-- PR links -
 
+[#1720]: https://github.com/munich-quantum-toolkit/core/pull/1720
 [#1719]: https://github.com/munich-quantum-toolkit/core/pull/1719
 [#1702]: https://github.com/munich-quantum-toolkit/core/pull/1702
 [#1694]: https://github.com/munich-quantum-toolkit/core/pull/1694

src/qdmi/driver/Driver.cpp

@@ -74,10 +74,14 @@ namespace {
 [[nodiscard]] auto loadDeviceLibrary(const std::string& libName) -> HMODULE {
   const auto requested = std::filesystem::path(libName);
 
-  const std::filesystem::path path = requested.has_parent_path()
-                                         ? requested
-                                         : getDriverDirectory() / requested;
+  if (requested.has_parent_path()) {
+    // A directory component was supplied: Directly load the library.
+    return LoadLibraryW(requested.wstring().c_str());
+  }
 
+  // Bare filename: resolve relative to the driver's own directory so that
+  // builtin device DLLs installed next to the driver are found reliably.
+  const auto path = getDriverDirectory() / requested;
   return LoadLibraryExW(path.wstring().c_str(), nullptr,
                         LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR |
                             LOAD_LIBRARY_SEARCH_DEFAULT_DIRS);