[Coverage] BuildBot.Discord — increase code coverage to 100%

[credfeto]

Problem

BuildBot.Discord has 0% line coverage and 0% branch coverage. There are no tests for any of the types in this assembly.

Closes part of #356.

Current State

• Line coverage: 0%
• Branch coverage: 0%

Files Requiring Tests

All source files in src/BuildBot.Discord/:

• BotMessage.cs
• BotReleaseMessage.cs
• BotService.cs
• DiscordBot.cs
• DiscordBotConfiguration.cs
• DiscordBotLoggingExtensions.cs
• DiscordBotMessageNotificationHandler.cs
• DiscordBotMessageNotificationHandlerLoggingExtensions.cs
• DiscordBotReleaseMessageNotificationHandler.cs
• DiscordSetup.cs
• IDiscordBot.cs
• IMessageChannel.cs
• MessageChannel.cs

Acceptance Criteria

• A new BuildBot.Discord.Tests project exists.
• All source types in BuildBot.Discord are covered at 100% line and branch coverage.
• All tests pass in CI.

[dnyw4l3n13]

Before starting implementation, I have two questions about testability that affect whether 100% coverage is achievable with the current code structure.

Question 1 — DiscordBot and BotService refactoring scope

DiscordBot creates DiscordSocketClient directly in its constructor (no injection point), and BotService takes DiscordBot as a concrete parameter rather than IDiscordBot. This means several code paths cannot be covered without a live Discord connection:

• DiscordBot.StartAsync() — lines after _client.LoginAsync() would never execute if login fails (invalid/empty token throws before returning).
• DiscordBot.PublishCommonAsync() — the try block's success path (sending a message, logging sent) requires a real SocketTextChannel.
• DiscordBot.ReconnectAsync() — only triggered from the catch block in PublishCommonAsync, which itself requires a real channel.
• DiscordBot.Log*Async() private methods — only triggered by Discord's internal Log event.
• BotService.StartAsync() / BotService.StopAsync() — delegate to DiscordBot, but DiscordBot is not mockable at the BotService boundary.
• BotService private PublishMessageAsync overloads — triggered only by the Reactive observable once Discord is connected.

Question: Is refactoring in scope for this issue? Specifically, should we:

a) Extract a IDiscordSocketClient abstraction and inject it into DiscordBot, or
b) Change BotService to accept IDiscordBot plus an additional lifecycle interface (since IDiscordBot does not currently expose StartAsync/StopAsync), or
c) Accept that these paths are untestable as-is and exclude them from coverage measurements?

Question 2 — DiscordBotMessageNotificationHandler and DiscordBotReleaseMessageNotificationHandler registration

AddDiscord does not register DiscordBotMessageNotificationHandler or DiscordBotReleaseMessageNotificationHandler — they appear to be Mediator source-generator–registered handlers. Should the DI test for this assembly only verify the services that AddDiscord explicitly registers (IDiscordBot, IComponentStatus, IMessageChannel<>, BotService / IHostedService)?