feat: Implement QueryExecutor for executing queries using IQueryable

- Added a record `QueryExecutor<TEntity>` implementing `IQueryExecutor<TEntity>` for executing common query operations like First, Single, and ToList both synchronously and asynchronously.
- Methods leverage the IQueryable interface to perform database queries for entity types.

EnvelopeGenerator.Application/Contracts/SQLExecutor/IQueryExecutor.cs

@@ -0,0 +1,70 @@
+namespace EnvelopeGenerator.Application.Contracts.SQLExecutor;
+
+/// <summary>
+/// Provides methods for executing common Entity Framework queries on a given entity type.
+/// This interface abstracts away the direct usage of Entity Framework methods for querying data
+/// and provides asynchronous and synchronous operations for querying a collection or single entity.
+/// </summary>
+/// <typeparam name="TEntity">The type of the entity being queried.</typeparam>
+public interface IQueryExecutor<TEntity>
+{
+    /// <summary>
+    /// Asynchronously retrieves the first entity or a default value if no entity is found.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation. The task result contains the entity or a default value.</returns>
+    public Task<TEntity?> FirstOrDefaultAsync();
+
+    /// <summary>
+    /// Asynchronously retrieves a single entity or a default value if no entity is found.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation. The task result contains the entity or a default value.</returns>
+    public Task<TEntity?> SingleOrDefaultAsync();
+
+    /// <summary>
+    /// Asynchronously retrieves a list of entities.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation. The task result contains the list of entities.</returns>
+    public Task<IEnumerable<TEntity>> ToListAsync();
+
+    /// <summary>
+    /// Asynchronously retrieves the first entity. Throws an exception if no entity is found.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation. The task result contains the first entity.</returns>
+    public Task<TEntity> FirstAsync();
+
+    /// <summary>
+    /// Asynchronously retrieves a single entity. Throws an exception if no entity is found.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation. The task result contains the single entity.</returns>
+    public Task<TEntity> SingleAsync();
+
+    /// <summary>
+    /// Synchronously retrieves the first entity or a default value if no entity is found.
+    /// </summary>
+    /// <returns>The first entity or a default value.</returns>
+    public TEntity? FirstOrDefault();
+
+    /// <summary>
+    /// Synchronously retrieves a single entity or a default value if no entity is found.
+    /// </summary>
+    /// <returns>The single entity or a default value.</returns>
+    public TEntity? SingleOrDefault();
+
+    /// <summary>
+    /// Synchronously retrieves a list of entities.
+    /// </summary>
+    /// <returns>The list of entities.</returns>
+    public IEnumerable<TEntity> ToList();
+
+    /// <summary>
+    /// Synchronously retrieves the first entity. Throws an exception if no entity is found.
+    /// </summary>
+    /// <returns>The first entity.</returns>
+    public TEntity First();
+
+    /// <summary>
+    /// Synchronously retrieves a single entity. Throws an exception if no entity is found.
+    /// </summary>
+    /// <returns>The single entity.</returns>
+    public TEntity Single();
+}

EnvelopeGenerator.Infrastructure/QueryExecutor.cs

@@ -0,0 +1,34 @@
+using EnvelopeGenerator.Application.Contracts.SQLExecutor;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Infrastructure;
+
+public record QueryExecutor<TEntity>(IQueryable<TEntity> Queryable) : IQueryExecutor<TEntity>
+{
+    public TEntity First() => Queryable.First();
+
+    public Task<TEntity> FirstAsync() => Queryable.FirstAsync();
+
+    public TEntity? FirstOrDefault() => Queryable.FirstOrDefault();
+
+
+    public Task<TEntity?> FirstOrDefaultAsync() => Queryable.FirstOrDefaultAsync();
+
+
+    public TEntity Single() => Queryable.Single();
+
+
+    public Task<TEntity> SingleAsync() => Queryable.SingleAsync();
+
+
+    public TEntity? SingleOrDefault() => Queryable.SingleOrDefault();
+
+
+    public Task<TEntity?> SingleOrDefaultAsync() => Queryable.SingleOrDefaultAsync();
+
+
+    public IEnumerable<TEntity> ToList() => Queryable.ToList();
+
+
+    public async Task<IEnumerable<TEntity>> ToListAsync() => await Queryable.ToListAsync();
+}