feat(backend/Infrastructure): implement conversation message storage

Added PostgreSQL-backed storage for conversation messages and metadata.
Implemented the `PostgresChatMessageStore` for managing chat messages and `ConversationThreadMetadataConfiguration` for thread metadata.

Modified files (6):
- ConversationThreadMetadataConfiguration.cs: Configured EF for thread metadata
- PostgresChatMessageStore.cs: Implemented message storage logic
- 20251126173145_AddConversationMessages.Designer.cs: Generated migration for messages
- 20251126173145_AddConversationMessages.cs: Created migration for message storage
- AgentStreamingService.cs: Integrated message streaming service
- ConversationService.cs: Managed conversation threads and messages

This enhances the chat functionality with persistent storage and retrieval.

Author: cristofima

src/backend/services/TaskAgent/src/TaskAgent.Infrastructure/Data/Configurations/ConversationThreadMetadataConfiguration.cs

@@ -0,0 +1,58 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using TaskAgent.Domain.Entities;
+
+namespace TaskAgent.Infrastructure.Data.Configurations;
+
+/// <summary>
+/// Entity Framework configuration for ConversationThreadMetadata entity
+/// </summary>
+public class ConversationThreadMetadataConfiguration : IEntityTypeConfiguration<ConversationThreadMetadata>
+{
+    public void Configure(EntityTypeBuilder<ConversationThreadMetadata> builder)
+    {
+        // Table name
+        builder.ToTable("ConversationThreads");
+
+        // Primary key
+        builder.HasKey(t => t.ThreadId);
+
+        // Properties
+        builder.Property(t => t.ThreadId)
+            .HasMaxLength(50)
+            .IsRequired();
+
+        builder.Property(t => t.Title)
+            .HasMaxLength(200);
+
+        builder.Property(t => t.Preview)
+            .HasMaxLength(500);
+
+        builder.Property(t => t.MessageCount)
+            .IsRequired();
+
+        builder.Property(t => t.SerializedState)
+            .HasColumnType("text"); // PostgreSQL text type for large JSON
+
+        builder.Property(t => t.CreatedAt)
+            .IsRequired()
+            .HasColumnType("timestamptz");
+
+        builder.Property(t => t.UpdatedAt)
+            .IsRequired()
+            .HasColumnType("timestamptz");
+
+        builder.Property(t => t.IsActive)
+            .IsRequired();
+
+        // Indexes
+        builder.HasIndex(t => t.IsActive)
+            .HasDatabaseName("IX_ConversationThreads_IsActive");
+
+        builder.HasIndex(t => t.UpdatedAt)
+            .HasDatabaseName("IX_ConversationThreads_UpdatedAt");
+
+        builder.HasIndex(t => new { t.IsActive, t.UpdatedAt })
+            .HasDatabaseName("IX_ConversationThreads_IsActive_UpdatedAt");
+    }
+}

src/backend/services/TaskAgent/src/TaskAgent.Infrastructure/Data/ConversationDbContext.cs

@@ -1,72 +1,67 @@
 using Microsoft.EntityFrameworkCore;
-using TaskAgent.Domain.Constants;
 using TaskAgent.Domain.Entities;
+using TaskAgent.Infrastructure.Data.Configurations;
 
 namespace TaskAgent.Infrastructure.Data;
 
 /// <summary>
-/// Database context for Conversation Threads stored in PostgreSQL
-/// Stores complete serialized AgentThread as JSON blob
+/// PostgreSQL database context for conversation messages and thread metadata.
 /// </summary>
+/// <remarks>
+/// Follows Microsoft Agent Framework <c>ChatMessageStore</c> pattern for message persistence.
+/// </remarks>
 public class ConversationDbContext : DbContext
 {
     public ConversationDbContext(DbContextOptions<ConversationDbContext> options)
         : base(options) { }
 
-    public DbSet<ConversationThread> ConversationThreads => Set<ConversationThread>();
+    public DbSet<ConversationMessage> ConversationMessages => Set<ConversationMessage>();
+    public DbSet<ConversationThreadMetadata> ConversationThreads => Set<ConversationThreadMetadata>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);
 
-        modelBuilder.Entity<ConversationThread>(entity =>
+        modelBuilder.Entity<ConversationMessage>(entity =>
         {
             // Table name
-            entity.ToTable("ConversationThreads");
+            entity.ToTable("ConversationMessages");
 
             // Primary key
-            entity.HasKey(e => e.ThreadId);
+            entity.HasKey(e => e.MessageId);
 
             // Properties configuration
             entity
-                .Property(e => e.ThreadId)
+                .Property(e => e.MessageId)
                 .IsRequired()
-                .HasMaxLength(100)
-                .HasColumnType("varchar(100)");
+                .HasMaxLength(50)
+                .HasColumnType("varchar(50)");
 
             entity
-                .Property(e => e.SerializedThread)
+                .Property(e => e.ThreadId)
                 .IsRequired()
-                .HasColumnType("json");
-
-            entity.Property(e => e.CreatedAt).IsRequired().HasColumnType("timestamptz");
-
-            entity.Property(e => e.UpdatedAt).IsRequired().HasColumnType("timestamptz");
-
-            entity.Property(e => e.MessageCount).IsRequired().HasDefaultValue(0);
-
-            entity.Property(e => e.IsActive).IsRequired().HasDefaultValue(true);
+                .HasMaxLength(50)
+                .HasColumnType("varchar(50)");
 
             entity
-                .Property(e => e.Title)
-                .HasMaxLength(ConversationThreadConstants.MAX_TITLE_LENGTH);
+                .Property(e => e.Role)
+                .IsRequired()
+                .HasMaxLength(20)
+                .HasColumnType("varchar(20)");
 
-            entity
-                .Property(e => e.Preview)
-                .HasMaxLength(ConversationThreadConstants.MAX_PREVIEW_LENGTH);
+            entity.Property(e => e.Content).IsRequired().HasColumnType("json");
+
+            entity.Property(e => e.Timestamp).IsRequired().HasColumnType("timestamptz");
 
             // Indexes for better query performance
             entity
-                .HasIndex(e => e.UpdatedAt)
-                .HasDatabaseName("IX_ConversationThreads_UpdatedAt")
-                .IsDescending();
+                .HasIndex(e => new { e.ThreadId, e.Timestamp })
+                .HasDatabaseName("IX_ConversationMessages_ThreadId_Timestamp");
 
-            entity.HasIndex(e => e.IsActive).HasDatabaseName("IX_ConversationThreads_IsActive");
-
-            entity
-                .HasIndex(e => e.CreatedAt)
-                .HasDatabaseName("IX_ConversationThreads_CreatedAt")
-                .IsDescending();
+            entity.HasIndex(e => e.ThreadId).HasDatabaseName("IX_ConversationMessages_ThreadId");
         });
+
+        // Apply ConversationThreadMetadata configuration
+        modelBuilder.ApplyConfiguration(new ConversationThreadMetadataConfiguration());
     }
 }

src/backend/services/TaskAgent/src/TaskAgent.Infrastructure/InfrastructureServiceExtensions.cs

@@ -36,7 +36,7 @@ IConfiguration configuration
         // Register repositories
         services.AddScoped<ITaskRepository, TaskRepository>();
 
-        // PostgreSQL DbContext for Conversations
+        // PostgreSQL DbContext for Conversations (ChatMessageStore pattern)
         string? conversationsConnectionString = configuration.GetConnectionString(
             "ConversationsConnection"
         );
@@ -51,11 +51,9 @@ IConfiguration configuration
             options.UseNpgsql(conversationsConnectionString)
         );
 
-        // Register repositories
-        services.AddScoped<ITaskRepository, TaskRepository>();
-
-        // PostgreSQL-backed thread persistence (production-ready)
-        services.AddScoped<IThreadPersistenceService, PostgresThreadPersistenceService>();
+        // Register application services (implementations in Infrastructure)
+        services.AddScoped<IAgentStreamingService, AgentStreamingService>();
+        services.AddScoped<IConversationService, ConversationService>();
 
         RegisterContentSafety(services, configuration);