Add integration tests for migration categories and execution

- Implemented MigrationCategoryTests to validate migration categorization for startup, release, seed, and data migrations.
- Added tests for edge cases, including null, empty, and whitespace migration names.
- Created StartupMigrationHostTests to verify the behavior of the migration host with real PostgreSQL instances using Testcontainers.
- Included tests for migration execution, schema creation, and handling of pending release migrations.
- Added SQL migration files for testing: creating a test table, adding a column, a release migration, and seeding data.

src/Concelier/StellaOps.Concelier.WebService/DualWrite/DualWriteAdvisoryStore.cs

@@ -0,0 +1,72 @@
+using Microsoft.Extensions.Logging;
+using MongoDB.Driver;
+using StellaOps.Concelier.Models;
+using StellaOps.Concelier.Storage.Mongo.Advisories;
+using StellaOps.Concelier.Storage.Postgres.Advisories;
+
+namespace StellaOps.Concelier.WebService.DualWrite;
+
+/// <summary>
+/// Dual-write advisory store that writes to both MongoDB and PostgreSQL simultaneously.
+/// Used during migration to verify parity between backends.
+/// </summary>
+/// <remarks>
+/// MongoDB is the primary store; PostgreSQL writes are best-effort with error logging.
+/// Read operations are always served from MongoDB.
+/// </remarks>
+public sealed class DualWriteAdvisoryStore : IAdvisoryStore
+{
+    private readonly AdvisoryStore _mongoStore;
+    private readonly IPostgresAdvisoryStore _postgresStore;
+    private readonly ILogger<DualWriteAdvisoryStore> _logger;
+
+    public DualWriteAdvisoryStore(
+        AdvisoryStore mongoStore,
+        IPostgresAdvisoryStore postgresStore,
+        ILogger<DualWriteAdvisoryStore> logger)
+    {
+        _mongoStore = mongoStore ?? throw new ArgumentNullException(nameof(mongoStore));
+        _postgresStore = postgresStore ?? throw new ArgumentNullException(nameof(postgresStore));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <inheritdoc />
+    public async Task UpsertAsync(Advisory advisory, CancellationToken cancellationToken, IClientSessionHandle? session = null)
+    {
+        // Write to MongoDB (primary)
+        await _mongoStore.UpsertAsync(advisory, cancellationToken, session).ConfigureAwait(false);
+
+        // Write to PostgreSQL (secondary, best-effort)
+        try
+        {
+            await _postgresStore.UpsertAsync(advisory, sourceId: null, cancellationToken).ConfigureAwait(false);
+            _logger.LogDebug("Dual-write success for advisory {AdvisoryKey}", advisory.AdvisoryKey);
+        }
+        catch (Exception ex)
+        {
+            // Log but don't fail - MongoDB is primary during migration
+            _logger.LogWarning(ex, "Dual-write to PostgreSQL failed for advisory {AdvisoryKey}. MongoDB write succeeded.", advisory.AdvisoryKey);
+        }
+    }
+
+    /// <inheritdoc />
+    public Task<Advisory?> FindAsync(string advisoryKey, CancellationToken cancellationToken, IClientSessionHandle? session = null)
+    {
+        // Always read from MongoDB during dual-write mode
+        return _mongoStore.FindAsync(advisoryKey, cancellationToken, session);
+    }
+
+    /// <inheritdoc />
+    public Task<IReadOnlyList<Advisory>> GetRecentAsync(int limit, CancellationToken cancellationToken, IClientSessionHandle? session = null)
+    {
+        // Always read from MongoDB during dual-write mode
+        return _mongoStore.GetRecentAsync(limit, cancellationToken, session);
+    }
+
+    /// <inheritdoc />
+    public IAsyncEnumerable<Advisory> StreamAsync(CancellationToken cancellationToken, IClientSessionHandle? session = null)
+    {
+        // Always read from MongoDB during dual-write mode
+        return _mongoStore.StreamAsync(cancellationToken, session);
+    }
+}