chore: organize solution into module folders (Core/Server/Drivers/Client/Tooling)

Group all 69 projects into category subfolders under src/ and tests/ so the
Rider Solution Explorer mirrors the module structure. Folders: Core, Server,
Drivers (with a nested Driver CLIs subfolder), Client, Tooling.

- Move every project folder on disk with git mv (history preserved as renames).
- Recompute relative paths in 57 .csproj files: cross-category ProjectReferences,
  the lib/ HintPath+None refs in Driver.Historian.Wonderware, and the external
  mxaccessgw refs in Driver.Galaxy and its test project.
- Rebuild ZB.MOM.WW.OtOpcUa.slnx with nested solution folders.
- Re-prefix project paths in functional scripts (e2e, compliance, smoke SQL,
  integration, install).

Build green (0 errors); unit tests pass. Docs left for a separate pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/GalaxyDriverApiKeyResolverTests.cs

@@ -0,0 +1,88 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests;
+
+/// <summary>
+///     Follow-up #2 — pins the three resolution forms supported by
+///     <see cref="GalaxyDriver.ResolveApiKey"/>: <c>env:NAME</c>, <c>file:PATH</c>,
+///     and the literal-string fallback. A future DPAPI arm slots in here without
+///     touching the call site.
+/// </summary>
+public sealed class GalaxyDriverApiKeyResolverTests
+{
+    [Fact]
+    public void Literal_string_is_returned_unchanged()
+    {
+        GalaxyDriver.ResolveApiKey("plain-text-key").ShouldBe("plain-text-key");
+    }
+
+    [Fact]
+    public void Env_prefix_resolves_to_environment_variable()
+    {
+        const string name = "OTOPCUA_TEST_GALAXY_API_KEY";
+        Environment.SetEnvironmentVariable(name, "key-from-env");
+        try
+        {
+            GalaxyDriver.ResolveApiKey($"env:{name}").ShouldBe("key-from-env");
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(name, null);
+        }
+    }
+
+    [Fact]
+    public void Env_prefix_unset_variable_throws_with_descriptive_message()
+    {
+        const string name = "OTOPCUA_TEST_GALAXY_API_KEY_UNSET";
+        Environment.SetEnvironmentVariable(name, null);
+
+        var ex = Should.Throw<InvalidOperationException>(() =>
+            GalaxyDriver.ResolveApiKey($"env:{name}"));
+        ex.Message.ShouldContain(name);
+        ex.Message.ShouldContain("unset");
+    }
+
+    [Fact]
+    public void File_prefix_resolves_to_trimmed_file_contents()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"galaxy-key-{Guid.NewGuid():N}.txt");
+        File.WriteAllText(path, "  key-from-file  \n");
+        try
+        {
+            GalaxyDriver.ResolveApiKey($"file:{path}").ShouldBe("key-from-file");
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+
+    [Fact]
+    public void File_prefix_missing_path_throws()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"does-not-exist-{Guid.NewGuid():N}.txt");
+        var ex = Should.Throw<InvalidOperationException>(() =>
+            GalaxyDriver.ResolveApiKey($"file:{path}"));
+        ex.Message.ShouldContain(path);
+        ex.Message.ShouldContain("doesn't exist");
+    }
+
+    [Fact]
+    public void File_prefix_empty_file_throws()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"galaxy-key-empty-{Guid.NewGuid():N}.txt");
+        File.WriteAllText(path, "   \n  ");
+        try
+        {
+            var ex = Should.Throw<InvalidOperationException>(() =>
+                GalaxyDriver.ResolveApiKey($"file:{path}"));
+            ex.Message.ShouldContain("empty");
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+}