Add old_text/new_text as a zeta2 prompt format (#42171)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>
Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>
Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/cloud_llm_client/src/predict_edits_v3.rs

@@ -73,6 +73,7 @@ pub enum PromptFormat {
     MarkedExcerpt,
     LabeledSections,
     NumLinesUniDiff,
+    OldTextNewText,
     /// Prompt format intended for use via zeta_cli
     OnlySnippets,
 }
@@ -100,6 +101,7 @@ impl std::fmt::Display for PromptFormat {
             PromptFormat::LabeledSections => write!(f, "Labeled Sections"),
             PromptFormat::OnlySnippets => write!(f, "Only Snippets"),
             PromptFormat::NumLinesUniDiff => write!(f, "Numbered Lines / Unified Diff"),
+            PromptFormat::OldTextNewText => write!(f, "Old Text / New Text"),
         }
     }
 }