fix: add support for ACP writeTextFile clientCapability

[ncwade]

This is an attempt to fix the issue called out in issue 4240. By mirroring the write back to the client it causes the local review flows to trigger. It feels a bit inefficient, but it seems to match with the expectations of the spec. I looked at hijacking the tool call to just do the writes and reads through the client, but I couldn't find a good pattern to make that fit.