zeta2: Support qwen3-minimal prompt format (#42902)

This prompt is for a fine-tuned model. It has the following changes,
compared to `minimal`:
- No instructions at all, except for one sentence at the beginning of
the prompt.
- Output is a simplified unified diff -- hunk headers have no line
counts (e.g., `@@ -20 +20 @@`)
- Qwen's FIM tokens are used where possible (`<|file_sep|>`,
`<|fim_prefix|>`, `<|fim_suffix|>`, etc.)

To evaluate this model:
```
ZED_ZETA2_MODEL=zeta2-exp [usual zeta-cli eval params ...]  --prompt-format minimal-qwen
```

This will point to the most recent Baseten deployment of zeta2-exp
(which may change in the future, so the prompt-format may get out of
sync).

Release Notes:

- N/A

crates/cloud_llm_client/src/predict_edits_v3.rs

@@ -78,6 +78,8 @@ pub enum PromptFormat {
     OnlySnippets,
     /// One-sentence instructions used in fine-tuned models
     Minimal,
+    /// One-sentence instructions + FIM-like template
+    MinimalQwen,
 }
 
 impl PromptFormat {
@@ -105,6 +107,7 @@ impl std::fmt::Display for PromptFormat {
             PromptFormat::NumLinesUniDiff => write!(f, "Numbered Lines / Unified Diff"),
             PromptFormat::OldTextNewText => write!(f, "Old Text / New Text"),
             PromptFormat::Minimal => write!(f, "Minimal"),
+            PromptFormat::MinimalQwen => write!(f, "Minimal + Qwen FIM"),
         }
     }
 }