fix: respect resource mime type in responses

The server was ignoring mime types set on resources, defaulting to text/plain
for strings and application/octet-stream for bytes. Now properly preserves
the specified mime type in both FastMCP and low-level server implementations.

Note that this is breaks backwards compatibility as it changes the return
values of read_resource() on FastMCP. It is BC compatible on lowlevel since
it only extends the callback.

Github-Issue: #152
Reported-by: eiseleMichael

tests/issues/test_152_resource_mime_type.py

@@ -98,9 +98,9 @@ async def test_lowlevel_resource_mime_type():
     @server.read_resource()
     async def handle_read_resource(uri: AnyUrl):
         if str(uri) == "test://image":
-            return base64_string
+            return (base64_string, "image/png")
         elif str(uri) == "test://image_bytes":
-            return image_bytes
+            return (bytes(image_bytes), "image/png")
         raise Exception(f"Resource not found: {uri}")
 
     # Test that resources are listed with correct mime type