refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Downloaders/DownloadContextBuilder.cs

@@ -0,0 +1,118 @@
+using Beam.Abstractions;
+using Beam.Models;
+using HtmlAgilityPack;
+using Microsoft.Extensions.Logging;
+
+namespace Beam.Downloaders {
+
+    public class DownloadContextBuilder<RawType> {
+        private HtmlWeb _web;
+        private HttpClient _client;
+        private IProgress<IDownloadReport>? _downloadReporter;
+        private IProgress<IRetryReport>? _retryReporter;
+        private AsyncDownloadFailurePredicate<RawType>?[] _asyncFailurePredicates = [];
+        private TimeSpan _timeOut;
+        private IEnumerable<string> _links;
+        private CancellationToken _cancellationToken;
+        private DocumentCache _cache;
+        private ILogger? _downloadLogger;
+
+        public DownloadContextBuilder(HttpClient? client = null, HtmlWeb? web = null) {
+            // You can initialize defaults here if needed, e.g.:
+            // _timeOut = TimeSpan.FromSeconds(30);
+            // _cancellationToken = CancellationToken.None;
+            _client = client ?? new();
+            _web = web ?? new();
+            _links = [];
+        }
+
+        public DownloadContextBuilder<RawType> WithWeb(HtmlWeb web) {
+            _web = web;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithClient(HttpClient client) {
+            _client = client;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithDownloadReporter(IProgress<IDownloadReport> downloadReporter) {
+            _downloadReporter = downloadReporter;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithRetryReporter(IProgress<IRetryReport> retryReporter) {
+            _retryReporter = retryReporter;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithAsyncFailurePredicates(params AsyncDownloadFailurePredicate<RawType>[] predicates) {
+            _asyncFailurePredicates = predicates;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithTimeOut(TimeSpan timeOut) {
+            _timeOut = timeOut;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithLinks(IEnumerable<string> links) {
+            _links = links;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithCancellationToken(CancellationToken cancellationToken) {
+            _cancellationToken = cancellationToken;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithCache(DocumentCache cache) {
+            _cache = cache;
+            return this;
+        }
+
+        public DownloadContextBuilder<RawType> WithDownloadLogger(ILogger downloadLogger) {
+            _downloadLogger = downloadLogger;
+            return this;
+        }
+
+
+        public DownloadContext<RawType> Build() {
+            // Construct the DownloadContext<T> using the collected values.
+            var context = new DownloadContext<RawType>(
+                web: _web,
+                client: _client,
+                links: _links,
+                cancellationToken: _cancellationToken,
+                downloadReporter: _downloadReporter,
+                retryReporter: _retryReporter,
+                asyncFailurePredicates: _asyncFailurePredicates,
+                timeOut: _timeOut,
+                downloadLogger: _downloadLogger
+            );
+
+            //// Assign the DocumentCache if it's been set in the builder.
+            //// (Even though Cache has a private setter, this code assumes builder
+            ////  is in the same assembly or that the setter will be made internal.
+            ////  Otherwise, remove or adjust this line.)
+            //context.Cache = _cache;
+
+            return context;
+        }
+
+        public static DownloadContextBuilder<RawType> FromContext(DownloadContext<RawType> existing) {
+            if (existing == null) throw new ArgumentNullException(nameof(existing));
+
+            return new DownloadContextBuilder<RawType>(existing.Client, existing.Web)
+                .WithLinks(existing.Links)
+                .WithCancellationToken(existing.CancellationToken)
+                .WithDownloadReporter(existing.DownloadReporter!)
+                .WithRetryReporter(existing.RetryReporter!)
+                .WithAsyncFailurePredicates(existing.AsyncFailurePredicates ?? Array.Empty<AsyncDownloadFailurePredicate<RawType>>())
+                .WithTimeOut(existing.TimeOut)
+                .WithDownloadLogger(existing.DownloadLogger!)
+                .WithCache(existing.Cache);
+        }
+    }
+
+}