refactor(export): read text from chunks, write the file atomically

Two changes to MessageExporter before it gets a caller.

It read SenderSource and ContentSource, the raw SeStrings. TextValue on one
holding an auto-translate phrase reaches SeStringEvaluator, which asserts it is
on the main thread and throws unconditionally when a macro resolves a global
number. An export belongs on a worker, so that would abort it partway and leave
half a file.

The plan called for resolving text in batches on the framework thread. Not
needed: Message.Sender and Message.Content are already-resolved chunk lists --
ChunkUtil turns auto-translate into text at ingest, and the full-text index reads
them exactly this way. Same strings, no evaluator, no thread affinity, and no
batching machinery.

Second, the file handling. The format was validated after the StreamWriter was
opened, so an unknown format left a zero-byte file where a previous export had
been. It is checked first now, and the write goes to a .part file that is moved
into place at the end. A crash halfway used to leave a file that opens cleanly
and is quietly incomplete -- which on the path a GDPR access request goes out on
is worse than an obvious failure.

Almost none of this is reachable from the build suite: ExportToFile takes
IEnumerable<Message>, Message needs SeString, and xUnit cannot load Dalamud.dll
-- even an empty list fails, because the runtime resolves the parameter type
before the body runs. So the format mapping is pinned there and the rest by a new
self-test, which builds probe messages with deliberately empty SeStrings: if the
exporter ever reads them again, the text comes out blank and it fails.

HellionChat/Plugin.cs

@@ -422,6 +422,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 await _lifecycle.LoadAsync(cancellationToken).ConfigureAwait(false);
 
             SelfTestRegistry.RegisterTestSteps([
+                new SelfTests.ExportRoundTripStep(),
                 new SelfTests.ThemeSwitchSelfTestStep(this),
                 new SelfTests.ThemeCrossfadeSelfTestStep(this),
                 new SelfTests.FontManagerCtorSmokeStep(this),