Fix pre-commit hook failure when package source mapping is active

The hook installs dotnet-format with --add-source, which NuGet rejects
when packageSourceMapping is configured. The dotnet10-transport feed is
already declared in NuGet.config; adding a dotnet-format mapping entry
there lets the install succeed without --add-source.

Assisted-by: Claude:claude-sonnet-4-6:GitHub Copilot

BuildTools/pre-commit

@@ -28,7 +28,7 @@ esac
 DOTNET_PATH="$DOTNET_CACHE_ROOT/ICSharpCode/ILSpy/dotnet-format-$DOTNET_FORMAT_VERSION"
 if [ ! -d "$DOTNET_PATH" ]; then
  echo "Downloading dotnet-format $DOTNET_FORMAT_VERSION..."
- dotnet tool install --tool-path "$DOTNET_PATH" dotnet-format --version "$DOTNET_FORMAT_VERSION" --add-source "$DOTNET_FORMAT_SOURCE"
+ dotnet tool install --tool-path "$DOTNET_PATH" dotnet-format --version "$DOTNET_FORMAT_VERSION"
 fi
 
 DOTNET_FORMAT="$DOTNET_PATH/dotnet-format$EXE_SUFFIX"

NuGet.config

@@ -16,6 +16,7 @@
     </packageSource>
     <packageSource key="dotnet10-transport">
       <package pattern="ILCompiler.Reflection.ReadyToRun.Experimental" />
+      <package pattern="dotnet-format" />
     </packageSource>
     <packageSource key="dotnet-tools">
       <package pattern="Microsoft.DiaSymReader.Converter.Xml" />