Migrate to .slnx solution format exclusively (#793)

## Overview
This PR migrates the repository from the traditional `.sln` format to
the modern `.slnx` solution file format exclusively, removing the legacy
`.sln` file to avoid CI/CD conflicts.

## Changes
- Generated `EssentialCSharp.Web.slnx` using `dotnet sln migrate`
command
- Removed the traditional `EssentialCSharp.Web.sln` file to prevent
"multiple solution files" errors in CI
- Updated Dockerfile to reference `.slnx` instead of `.sln`

## Why This Change?
The `.slnx` format is the modern solution file format introduced in
recent .NET SDK versions. It provides:
- **Cleaner format**: 19 lines vs 63 lines (846 bytes vs 3.8KB)
- **Better readability**: Human-friendly XML structure
- **Forward compatibility**: Prepares the repository for future .NET
tooling that may prefer or require this format
- **CI/CD compatibility**: Having only one solution file prevents
ambiguity errors when dotnet commands don't specify which solution to
use

By using only the `.slnx` format, we eliminate the MSB1011 error that
occurred when multiple solution files were present in the same
directory.

## Verification
The .slnx solution file has been verified to work correctly:
- ✅ `dotnet restore` automatically finds and uses the `.slnx` file
- ✅ `dotnet build` works without specifying a solution file
- ✅ `dotnet test` runs successfully (54/55 tests pass - one pre-existing
network connectivity test failure)
- ✅ Dockerfile updated to explicitly reference `.slnx` for Docker builds

## References
- [Microsoft Documentation: dotnet sln
migrate](https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-sln#migrate)

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> run `dotnet sln migrate`
(https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-sln#migrate)
to add in a .slnx file to live alongside our .sln file for now (.sln for
backwards compatibility for now)


</details>


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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BenjaminMichaelis <[email protected]>

Author: Copilot

EssentialCSharp.Web.sln

@@ -0,0 +1,19 @@
+<Solution>
+  <Folder Name="/Solution Items/">
+    <File Path="../.gitattributes" />
+    <File Path="../azure-pipelines.yml" />
+    <File Path=".editorconfig" />
+    <File Path="Directory.Build.props" />
+    <File Path="Directory.Build.targets" />
+    <File Path="Directory.Packages.props" />
+    <File Path="EssentialCSharp.Web/Dockerfile" />
+    <File Path="global.json" />
+    <File Path="nuget.config" />
+    <File Path="README.md" />
+  </Folder>
+  <Project Path="EssentialCSharp.Chat.Shared/EssentialCSharp.Chat.Common.csproj" />
+  <Project Path="EssentialCSharp.Chat.Tests/EssentialCSharp.Chat.Tests.csproj" />
+  <Project Path="EssentialCSharp.Chat/EssentialCSharp.Chat.csproj" />
+  <Project Path="EssentialCSharp.Web.Tests/EssentialCSharp.Web.Tests.csproj" />
+  <Project Path="EssentialCSharp.Web/EssentialCSharp.Web.csproj" />
+</Solution>

EssentialCSharp.Web.slnx

@@ -16,9 +16,9 @@ RUN --mount=type=secret,id=nuget_auth_token \
       auth_token=$(cat /run/secrets/nuget_auth_token) && \
       export VSS_NUGET_EXTERNAL_FEED_ENDPOINTS="{\"endpointCredentials\": [{\"endpoint\":\"https://pkgs.dev.azure.com/intelliTect/_packaging/EssentialCSharp/nuget/v3/index.json\", \"password\":\"$auth_token\"}]}"; \
     fi && \
-    dotnet restore "EssentialCSharp.Web.sln" -p:AccessToNugetFeed=$ACCESS_TO_NUGET_FEED && \
-    dotnet build "EssentialCSharp.Web.sln" -c Release --no-restore -p:AccessToNugetFeed=$ACCESS_TO_NUGET_FEED -p:ReleaseDateAttribute=True && \
-    dotnet publish "EssentialCSharp.Web.sln" -c Release -p:PublishDir=/app/publish -p:UseAppHost=false --no-build
+    dotnet restore "EssentialCSharp.Web.slnx" -p:AccessToNugetFeed=$ACCESS_TO_NUGET_FEED && \
+    dotnet build "EssentialCSharp.Web.slnx" -c Release --no-restore -p:AccessToNugetFeed=$ACCESS_TO_NUGET_FEED -p:ReleaseDateAttribute=True && \
+    dotnet publish "EssentialCSharp.Web.slnx" -c Release -p:PublishDir=/app/publish -p:UseAppHost=false --no-build
 
 FROM base AS final
 WORKDIR /app