Fix Uint8Array to BlobPart type compatibility for TypeScript 5.9

TypeScript 5.9.2 (shipped with Deno 2.6.3) no longer considers
Uint8Array directly assignable to BlobPart. This fixes the type error
by extracting the underlying ArrayBuffer with slice() before creating
the Blob.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dahlia

packages/jmap/src/jmap-transport.ts

@@ -398,7 +398,12 @@ export class JmapTransport implements Transport {
       const content = await attachment.content;
 
       // Create a Blob from the Uint8Array
-      const blob = new Blob([content], { type: attachment.contentType });
+      // Extract the ArrayBuffer portion to ensure TypeScript compatibility
+      const arrayBuffer = content.buffer.slice(
+        content.byteOffset,
+        content.byteOffset + content.byteLength,
+      ) as ArrayBuffer;
+      const blob = new Blob([arrayBuffer], { type: attachment.contentType });
 
       const result = await uploadBlob(
         this.config,