Add query/handler to fetch third-party module license text

Introduced ReadThirdPartyModuleLicenseQuery and its handler to retrieve the license text of a third-party module by Id or Name (mutually exclusive), with optional filtering for active modules. Includes validation and exception handling for invalid input, not found, or multiple matches.

EnvelopeGenerator.Application/ThirdPartyModules/Queries/ReadThirdPartyModuleLicenseQuery.cs

@@ -0,0 +1,92 @@
+using DigitalData.Core.Abstraction.Application.Repository;
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.ThirdPartyModules.Queries;
+
+/// <summary>
+/// Query to read the license text of a third-party module.
+/// Either <see cref="Id"/> or <see cref="Name"/> must be provided, but not both.
+/// </summary>
+public record ReadThirdPartyModuleLicenseQuery : IRequest<string>
+{
+    /// <summary>
+    /// The unique identifier of the third-party module (optional).
+    /// </summary>
+    public int? Id { get; init; }
+
+    /// <summary>
+    /// The name of the third-party module (optional).
+    /// </summary>
+    public string? Name { get; init; }
+
+    /// <summary>
+    /// Whether to filter only active modules. Defaults to <c>true</c>.
+    /// </summary>
+    public bool Active { get; init; } = true;
+}
+
+/// <summary>
+/// Handles <see cref="ReadThirdPartyModuleLicenseQuery"/> and returns the license text of a third-party module.
+/// </summary>
+public class ReadThirdPartyModuleLicenseQueryHandler : IRequestHandler<ReadThirdPartyModuleLicenseQuery, string>
+{
+    private readonly IRepository<ThirdPartyModule> _repository;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReadThirdPartyModuleLicenseQueryHandler"/> class.
+    /// </summary>
+    /// <param name="repository">The repository for accessing third-party modules.</param>
+    public ReadThirdPartyModuleLicenseQueryHandler(IRepository<ThirdPartyModule> repository)
+    {
+        _repository = repository;
+    }
+
+    /// <summary>
+    /// Handles the query by filtering on Id or Name and returning the license text.
+    /// </summary>
+    /// <param name="request">The query parameters.</param>
+    /// <param name="cancel">A cancellation token.</param>
+    /// <returns>The license text of the matching third-party module.</returns>
+    /// <exception cref="BadRequestException">
+    /// Thrown when neither Id nor Name is provided, or when both are provided.
+    /// </exception>
+    /// <exception cref="InvalidOperationException">
+    /// Thrown when multiple modules match the given criteria, indicating a data integrity issue.
+    /// </exception>
+    /// <exception cref="NotFoundException">
+    /// Thrown when no module matches the given criteria.
+    /// </exception>
+    public async Task<string> Handle(ReadThirdPartyModuleLicenseQuery request, CancellationToken cancel)
+    {
+        if (request.Id is null && request.Name is null)
+            throw new BadRequestException("Either Id or Name must be provided.");
+
+        if (request.Id is not null && request.Name is not null)
+            throw new BadRequestException("Only one of Id or Name must be provided, not both.");
+
+        var query = _repository.Query
+            .Where(m => m.Active == request.Active);
+
+        if (request.Id is int id)
+            query = query.Where(m => m.Id == id);
+
+        if (request.Name is string name)
+            query = query.Where(m => m.Name == name);
+
+        var modules = await query
+            .Take(2)
+            .ToListAsync(cancel);
+
+        if (modules.Count > 1)
+            throw new InvalidOperationException(
+                $"Data integrity violation: multiple third-party modules found for the given criteria (Id={request.Id}, Name={request.Name}, Active={request.Active}).");
+
+        return modules.SingleOrDefault() is ThirdPartyModule module
+            ? module.License
+            : throw new NotFoundException(
+                $"Third-party module not found for the given criteria (Id={request.Id}, Name={request.Name}, Active={request.Active}).");
+    }
+}