Remove QgsProject::instance() references from QgsPolymorphicRelation (QEP 423)

[JanCaha]

Description

Use project from QgsRelationContext, it's project() already handles situation when project would be empty to fallback to QgsProject:.instance(). So this change should be completely safe.

AI tool usage

• AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR.

[github-actions]

🪟 x64-windows — ❌ 1 port(s) failed

Failed: py-pandas

📦 py-pandas

_{from vcpkg-manifest-install.log (portfile-execute abort)}

  scripts/cmake/vcpkg_from_github.cmake:120 (z_vcpkg_download_distfile)
  C:/Users/runneradmin/AppData/Local/vcpkg/registries/git-trees/910f6fd1f988c66cd5c4ee3290ff174cdbd81cf7/portfile.cmake:19 (vcpkg_from_github)
  scripts/ports.cmake:209 (include)


error: building py-pandas:x64-windows-release failed with: BUILD_FAILED
See https://learn.microsoft.com/vcpkg/troubleshoot/build-failures?WT.mc_id=vcpkg_inproduct_cli for more information.
Elapsed time to handle py-pandas:x64-windows-release: 2.3 s
Please ensure you're using the latest port files with `git pull` and `vcpkg update`.
Then check for known issues at:
  https://github.com/microsoft/vcpkg/issues?q=is%3Aissue+is%3Aopen+in%3Atitle+py-pandas
You can submit a new issue at:
  https://github.com/microsoft/vcpkg/issues/new?title=%5Bpy-pandas%5D%20build%20error%20on%20x64-windows-release&body=Copy%20issue%20body%20from%20collapsed%20section%20%22issue_body.md%22%20in%20the%20ci%20log%20output
::group::issue_body.md
Package: py-pandas:x64-windows-release@3.0.5

**Host Environment**

- Host: x64-windows
- Compiler: MSVC 19.44.35228.0
- CMake Version: 4.4.0
-    vcpkg-tool version: 2026-07-27-98d7cb0cf1f4686a3e43aa5672b6230c1d56bce8
    vcpkg-scripts version: unknown

**To Reproduce**

`vcpkg install --x-feature=3d --x-feature=auth --x-feature=bindings --x-feature=exiv2 --x-feature=gui --x-feature=pdal --x-feature=qtpositioning --x-feature=sfcgal`

**Failure logs**

Added vcpkg bin path to search for dlls. Path added: D:\a\QGIS\QGIS\build\vcpkg_installed\x64-windows-release\bin
-- Found Python version '3.12.11 at D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/tools/python3/python.exe'
-- Using meson: C:/.vcpkg/downloads/tools/meson-1.9.0-633807/meson.py
Trying to download pandas-dev-pandas-v3.0.5.tar.gz using asset cache https://assetcache.open-vcpkg.org/assetcache/af22c3952ba5a96271ba07f947484bff82cc2d7d3c2bd53b60b48b7c201821ddacd2882d8d5b6c1b44273ae23207bead67483aa500f2596b867331d478304d05
Asset cache miss; trying authoritative source https://github.com/pandas-dev/pandas/archive/v3.0.5.tar.gz
error: curl operation failed with response code 404.
error: Not a transient network error, won't retry download from https://assetcache.open-vcpkg.org/assetcache/af22c3952ba5a96271ba07f947484bff82cc2d7d3c2bd53b60b48b7c201821ddacd2882d8d5b6c1b44273ae23207bead67483aa500f2596b867331d478304d05
pandas-dev-pandas-v3.0.5.tar.gz.1556.part: error: download from https://github.com/pandas-dev/pandas/archive/v3.0.5.tar.gz had an unexpected hash
note: Expected: af22c3952ba5a96271ba07f947484bff82cc2d7d3c2bd53b60b48b7c201821ddacd2882d8d5b6c1b44273ae23207bead67483aa500f2596b867331d478304d05

</details>

📦 [Download `failing-port-logs-x64-windows` artifact](https://github.com/qgis/QGIS/actions/runs/35225440842/artifacts/10500397956) &middot; 📑 [Download full `build-logs-x64-windows` artifact](https://github.com/qgis/QGIS/actions/runs/35225440842/artifacts/10500817184) &middot; 🔧 [Workflow run](https://github.com/qgis/QGIS/actions/runs/35225440842) &middot; 📌 [Commit `acdb32b`](https://github.com/qgis/QGIS/commit/acdb32bcf892a901e0973092a448bd272effe4e4)
<!-- ====endsection vcpkg-failures-x64-windows-->

<!-- ====startsection macos-qt6-->
### 🍎 MacOS Qt6 builds
Download [MacOS Qt6 builds of this PR for testing](https://github.com/qgis/QGIS/actions/runs/35225440667/artifacts/10521034872).
_This app is not notarized, run `sudo xattr -d com.apple.quarantine /Applications/QGIS*.app` to avoid the warning_
*(Built from commit acdb32bcf892a901e0973092a448bd272effe4e4)*

<!-- ====endsection macos-qt6-->