fix(copilot): match durable path — detect markdown by MIME type + name at the stream gate

The streaming gate checked isMarkdownFile with only the filename, while the durable merge
uses type + name — so a text/markdown file without a .md extension was skipped mid-stream
(it self-corrected at the durable write). Pass editIntent.contentType so streaming detects
the same set of markdown files as the durable path.

Author: waleedlatif1

apps/sim/lib/copilot/request/go/file-preview-adapter.ts

@@ -668,7 +668,7 @@ export async function processFilePreviewStreamEvent(input: {
           // no-op — so a slow relay can't backlog stale snapshots or make the doc lag the stream.
           const dueForLiveMerge =
             nextSession.fileId !== undefined &&
-            isMarkdownFile({ name: nextSession.fileName ?? '' }) &&
+            isMarkdownFile({ type: editIntent.contentType, name: nextSession.fileName ?? '' }) &&
             (editIntent.operation === 'append' || editIntent.operation === 'patch') &&
             currentPreview.session.baseContent !== undefined &&
             !isLiveDocMergeInFlight(nextSession.fileId) &&