…et#13245)

When using MSBuild's -mt mode (/maxcpucount), tasks without
[MSBuildMultiThreadableTask] are routed to out-of-process TaskHost
sidecars. Items passed to these tasks are serialized via
TaskParameterTaskItem, which calls CloneCustomMetadataEscaped() —
copying only custom metadata. RecursiveDir is a built-in metadata
that is non-derivable (requires _includeBeforeWildcardExpansionEscaped),
so it's lost during this serialization, returning empty string on
the TaskHost side.

This causes dotnet pack /mt to flatten directory structures in NuGet
packages (e.g., build/wix/bundle/bundle.wxs becomes build/bundle.wxs).

Fix: Explicitly copy RecursiveDir to custom metadata in the
TaskParameterTaskItem constructor before serialization, so it
survives the cross-process boundary.

Unlike the previous attempt (PR dotnet#13142, reverted in PR dotnet#13245),
this fix ONLY touches the TaskHost serialization boundary
(TaskParameterTaskItem). It does NOT change GatherTaskItemOutputs
or ProjectItemInstance, which was what caused the NuGet/sfxproj
double-append regression. Items that lose RecursiveDir at the
MSBuild task callback boundary (e.g., sfxproj targets returning
items via TargetOutputs) are unaffected — they already have
RecursiveDir=empty before reaching TaskParameterTaskItem.

Fixes dotnet#13140
Related: dotnet#3121

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>