This PR improves Zeta2's matching of `old_text`/`new_text` pairs, using
similar code to what we use in the edit agent. For right now, we've
duplicated the code, as opposed to trying to generalize it.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>
Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Agus <agus@zed.dev>