assistant: Refesh message headers only for dirty messages (#16881)

We've noticed performance issues in long conversations with assistants;
the profiles pointed to slowiness in WrapMap (and indeed there were some
low hanging fruits that we picked up in
https://github.com/zed-industries/zed/pull/16761). That however did not
fully resolve the issue, as WrapMap still cracked through in profiles;
basically, the speedup I've landed has just moved the post elsewhere.

The higher level issue is that we were trying to refresh message headers
for all messages, irrespective of whether they've actually needed to be
updated. This PR fixes that by using `replace_blocks` API where
possible.

Release Notes:

- Improved performance of Assistant Panel with long conversations.