fix(security): Add 3 packages.lock.json files for dependency locking (#299)

## .NET Lockfile Creation

This PR adds packages.lock.json files to ensure reproducible builds and
secure dependency management.

### Files Added:
- `sample/SendMailTestApp/packages.lock.json`
- `src/FluentEmail.Graph/packages.lock.json`
- `tests/FluentEmail.Graph.Tests/packages.lock.json`

### Benefits:
- **Reproducible builds**: Locked dependency versions ensure consistent
builds across environments
- **Security**: Prevents supply chain attacks by pinning specific
dependency versions
- **Performance**: Faster builds in CI/CD pipelines
- **Deterministic child dependency scanning**: Scanning for child
dependencies becomes deterministic, reducing false positives in
dependency analysis

The lockfiles were generated using `dotnet restore` command.

Co-authored-by: aikido-autofix[bot] <119856028+aikido-autofix[bot]@users.noreply.github.com>

Author: aikido-autofix[bot]

sample/SendMailTestApp/SendMailTestApp.csproj

@@ -7,6 +7,7 @@
         <Nullable>enable</Nullable>
       <UserSecretsId>4eba8463-a290-440f-b079-62d74334211a</UserSecretsId>
       <LangVersion>latestmajor</LangVersion>
+      <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
   </PropertyGroup>
 
     <ItemGroup>