[repo-assist] Fix #3209: preserve Paket.Restore.targets import position

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Closes #3209

Root cause

installForDotnetSDK (in src/Paket.Core/Installation/InstallProcess.fs) unconditionally called RemoveImportForPaketTargets() followed by AddImportForPaketTargets(...) on every paket install/restore, regardless of whether the correct <Import Project="...Paket.Restore.targets" /> node was already present. AddImportForPaketTargets always appends the new node at the end of the project file, so any project whose import was not already last got it moved there on every run.

Fix

Before removing/re-adding, check whether an Import node with the same relative path already exists. If it does, skip the remove/re-add entirely, leaving the existing node (and its position) untouched. This preserves import ordering for projects/tooling that depend on it, while still fixing/adding the import when it's missing or points at a different (stale) path.

Trade-offs

• Minimal, surgical change — no behavior change for the common case where the import is missing or already at the end.
• Did not add a dedicated unit test for this specific scenario due to the amount of scaffolding required to spin up a full ProjectFile/installForDotnetSDK test fixture; existing ProjectFile test suite (109 tests) still passes unmodified, confirming no regressions.

Test Status

• ✅ Build: dotnet build src/Paket.Core/Paket.Core.fsproj -f netstandard2.0 — succeeded (0 errors).
• ✅ Tests: dotnet test tests/Paket.Tests/Paket.Tests.fsproj -f net9 --filter "FullyQualifiedName~ProjectFile" — 109/109 passed.
• Full solution build/test not run due to sandbox network restrictions (Build dependency group needs ci.appveyor.com); Main-group restore and targeted netstandard2.0/net9 build+test were used instead (documented workaround from prior runs).

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/repo-assist Add a test for this

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✓ Repo Assist completed successfully, see workflow run.

Command mode: responded to /repo-assist "Add a test for this" on PR #4364. Added tests/Paket.Tests/ProjectFile/InstallForDotnetSDKSpecs.fs with 2 unit tests covering InstallProcess.installForDotnetSDK's #3209 fix (import position preserved when already present; import still added when missing), registered in Paket.Tests.fsproj, and pushed the commit to the PR branch via push_to_pull_request_branch. Verified: dotnet test --filter "FullyQualifiedName~ProjectFile" — 111/111 passed. Per command-mode instructions, exiting without running the normal scheduled workflow tasks.

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