Update targets, dependencies, and improve Library API

- Bump NuGet package versions for Azure, AWS, BouncyCastle, Google, Microsoft.Extensions, and xUnit
- Change target frameworks to net481 and net10.0
- Make Library class partial; add thread-safe explicit Initialize()
- Require initialization before encrypt/decrypt; throw if not initialized
- Add CancellationToken support to all async methods
- Zero key material from memory after use for security
- Use stricter key name validation (GeneratedRegex on .NET 8+)
- Expand and clarify XML documentation
- Improve exception handling and parameter validation
- Mark synchronous APIs as obsolete and document deadlock risk
- Minor code style and documentation improvements

Author: fkitsantas

Reina.Cryptography.Tests/Reina.Cryptography.Tests.csproj

@@ -10,10 +10,16 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="coverlet.collector" Version="6.0.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="xunit" Version="2.5.3" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+    <PackageReference Include="coverlet.collector" Version="10.0.0">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>