Add Core.ExternalBackend to Microsoft.Maui-dev.sln

The macOS CI legs build Microsoft.Maui-mac.slnf, and both solution filters
resolve against Microsoft.Maui-dev.sln - not Microsoft.Maui.sln, which is what
the Windows leg builds and what the new test project was added to.

Listing the project in the filters without adding it to the filtered solution
made solution load fail before restore:

  Microsoft.Maui-mac.slnf(1,1): error MSB4025: The project file could not be
  loaded. Data at the root level is invalid. Line 1, position 1.

Reproduced locally, where MSBuild reports the underlying cause directly:

  error MSB5028: Solution filter file at ".../Microsoft.Maui-mac.slnf" includes
  project "src/Core/tests/ExternalBackend/Core.ExternalBackend.csproj" that is
  not in the solution file at ".../Microsoft.Maui-dev.sln".

Adds the project to Microsoft.Maui-dev.sln under the same test solution folder
as Core.UnitTests. Both filters and both solutions now load cleanly.

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

Author: Redth

Microsoft.Maui-dev.sln

@@ -37,6 +37,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "TestUtils", "src\TestUtils\
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Core.UnitTests", "src\Core\tests\UnitTests\Core.UnitTests.csproj", "{92644F6F-5946-48FC-A21A-A3D6EE24E8B3}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Core.ExternalBackend", "src\Core\tests\ExternalBackend\Core.ExternalBackend.csproj", "{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4}"
+EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{E8AD265B-3C67-4640-AC58-A522F9FB3361}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{C564DDD6-DE79-45CD-88EA-3F690481572A}"
@@ -309,6 +311,10 @@ Global
 		{92644F6F-5946-48FC-A21A-A3D6EE24E8B3}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{92644F6F-5946-48FC-A21A-A3D6EE24E8B3}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{92644F6F-5946-48FC-A21A-A3D6EE24E8B3}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4}.Release|Any CPU.Build.0 = Release|Any CPU
 		{F7F2B379-52CE-4802-9EC9-0D7967B6BFB7}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{F7F2B379-52CE-4802-9EC9-0D7967B6BFB7}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{F7F2B379-52CE-4802-9EC9-0D7967B6BFB7}.Release|Any CPU.ActiveCfg = Release|Any CPU
@@ -679,6 +685,7 @@ Global
 		{DAAC2822-63B6-4DE0-83AE-04873CD2F364} = {72397ADB-40A8-4B8E-8E08-2DBE2803C845}
 		{FBB3270F-1924-4A72-845E-A6DF39C402F6} = {7AC28763-9C68-4BF9-A1BA-25CBFFD2D15C}
 		{92644F6F-5946-48FC-A21A-A3D6EE24E8B3} = {C564DDD6-DE79-45CD-88EA-3F690481572A}
+		{7B2E1C4D-9A5F-4E38-8D6B-2C0F1A73E9D4} = {C564DDD6-DE79-45CD-88EA-3F690481572A}
 		{E8AD265B-3C67-4640-AC58-A522F9FB3361} = {09C264E9-E3F3-4586-9151-DCBB1F6DA7AB}
 		{C564DDD6-DE79-45CD-88EA-3F690481572A} = {09C264E9-E3F3-4586-9151-DCBB1F6DA7AB}
 		{50C758FE-4E10-409A-94F5-A75480960864} = {459BF674-83CB-46F6-881F-A2D2117DBF4D}