Resolve thread mentions on demand (#1480)

* Initial plan

* Fix unresolved thread mentions in HTML export (#1261)

- Add TryGetChannelAsync to DiscordClient for on-demand channel/thread lookup
- Add PopulateChannelAsync to ExportContext with negative caching
- Update HtmlMarkdownVisitor to resolve thread mentions on demand

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Refactor GetChannelAsync to use TryGetChannelAsync for parent resolution

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Add test for thread mention resolution in HTML export

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Apply PopulateChannelAsync to PlainTextMarkdownVisitor; add JSON thread mention test

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Cli.Tests/Specs/HtmlMentionSpecs.cs

@@ -61,4 +61,17 @@ public class HtmlMentionSpecs
         // Assert
         message.Text().Should().Contain("Role mention: @Role 1");
     }
+
+    [Fact]
+    public async Task I_can_export_a_channel_that_contains_a_message_with_a_thread_mention()
+    {
+        // Act
+        var message = await ExportWrapper.GetMessageAsHtmlAsync(
+            ChannelIds.MentionTestCases,
+            Snowflake.Parse("1474874276828938290")
+        );
+
+        // Assert
+        message.Text().Should().Contain("Thread mention: #Thread starting message");
+    }
 }