In the case where the model would respond with `new_text` before
`old_text`, we would just emit an empty `old_text`, because the parsing
layer was operating under the assumption that `old_text` occurs before
`new_text`.

We now hold back new text chunks if we receive them first, and only emit
them once old_text is complete.

In addition to that we also need to handle the case where the first
chunk contains `old_text` and `new_text`. In that case we don't know
which one of the two fields have finished streaming, since we can't rely
on the ordering anymore. Therefore we hold back all events until we
receive the full edit, and emit a single OldTextChunk (done = true) and
a single NewTextChunk (done = true)

Closes #55398

Release Notes:

- agent: Fixed an issue where editing would sometimes fail for specific
models (Deepseek v4)