Add Essentials.ExternalBackend.UnitTests to Microsoft.Maui-dev.sln

Fixes the macOS Debug/Release build break introduced by the previous commit:

  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.

Microsoft.Maui-mac.slnf and Microsoft.Maui-windows.slnf both filter
Microsoft.Maui-dev.sln, not Microsoft.Maui.sln. The new test project was
only added to Microsoft.Maui.sln, so the filter referenced a project that
its backing solution did not contain. Reproduced locally as:

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

Add the project to Microsoft.Maui-dev.sln, and to Microsoft.Maui-vscode.sln
for consistency with Essentials.UnitTests, which is listed in all three
solutions.

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

Author: Redth

Microsoft.Maui-dev.sln

@@ -95,6 +95,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.DeviceTests", "s
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.UnitTests", "src\Essentials\test\UnitTests\Essentials.UnitTests.csproj", "{A9BAB840-A53E-405A-9458-CA0E02319FB4}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.ExternalBackend.UnitTests", "src\Essentials\test\ExternalBackendTests\Essentials.ExternalBackend.UnitTests.csproj", "{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}"
+EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Controls.Maps", "src\Controls\Maps\src\Controls.Maps.csproj", "{5C40FA00-7D48-4F54-898C-01C8942E451D}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{25D0D27A-C5FE-443D-8B65-D6C987F4A80E}"
@@ -379,6 +381,10 @@ Global
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Release|Any CPU.Build.0 = Release|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Release|Any CPU.ActiveCfg = Release|Any CPU
@@ -705,6 +711,7 @@ Global
 		{420BC8CC-EE70-49A1-964A-CB63414A882D} = {94F3C036-A5F4-4ACC-A028-8506802ADB88}
 		{66528BE6-FF53-4A3B-8675-46C0B929C83B} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
 		{5C40FA00-7D48-4F54-898C-01C8942E451D} = {50C758FE-4E10-409A-94F5-A75480960864}
 		{25D0D27A-C5FE-443D-8B65-D6C987F4A80E} = {459BF674-83CB-46F6-881F-A2D2117DBF4D}
 		{094FB109-1CBC-4B84-8CFB-072E7B3EC637} = {25D0D27A-C5FE-443D-8B65-D6C987F4A80E}

Microsoft.Maui-vscode.sln

@@ -93,6 +93,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.DeviceTests", "s
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.UnitTests", "src\Essentials\test\UnitTests\Essentials.UnitTests.csproj", "{A9BAB840-A53E-405A-9458-CA0E02319FB4}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Essentials.ExternalBackend.UnitTests", "src\Essentials\test\ExternalBackendTests\Essentials.ExternalBackend.UnitTests.csproj", "{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}"
+EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Controls.Maps", "src\Controls\Maps\src\Controls.Maps.csproj", "{5C40FA00-7D48-4F54-898C-01C8942E451D}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{25D0D27A-C5FE-443D-8B65-D6C987F4A80E}"
@@ -342,6 +344,10 @@ Global
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71}.Release|Any CPU.Build.0 = Release|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{5C40FA00-7D48-4F54-898C-01C8942E451D}.Release|Any CPU.ActiveCfg = Release|Any CPU
@@ -617,6 +623,7 @@ Global
 		{420BC8CC-EE70-49A1-964A-CB63414A882D} = {94F3C036-A5F4-4ACC-A028-8506802ADB88}
 		{66528BE6-FF53-4A3B-8675-46C0B929C83B} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
 		{A9BAB840-A53E-405A-9458-CA0E02319FB4} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
+		{7C6A0D6E-9B4E-4C0E-9E3C-2F1A5D8B4E71} = {420BC8CC-EE70-49A1-964A-CB63414A882D}
 		{5C40FA00-7D48-4F54-898C-01C8942E451D} = {50C758FE-4E10-409A-94F5-A75480960864}
 		{25D0D27A-C5FE-443D-8B65-D6C987F4A80E} = {459BF674-83CB-46F6-881F-A2D2117DBF4D}
 		{094FB109-1CBC-4B84-8CFB-072E7B3EC637} = {25D0D27A-C5FE-443D-8B65-D6C987F4A80E}