agent: Fix creating files with Gemini (#31439)

This change instructs models to wrap new file content in Markdown fences
and introduces a parser for this format. The reasons are:

1. This is the format we put a lot of effort into explaining in the
system prompt.
2. Gemini really prefers to do it.
3. It adds an option for a model to think before writing the content

The `eval_zode` pass rate for GEmini models goes from 0% to 100%. Other
models were already at 100%, this hasn't changed.


Release Notes:

- N/A

crates/assistant_tools/src/templates/create_file_prompt.hbs

@@ -1,8 +1,10 @@
 You are an expert engineer and your task is to write a new file from scratch.
 
-You MUST respond directly with the file's content, without explanations, additional text or triple backticks.
+You MUST respond with the file's content wrapped in triple backticks (```).
+The backticks should be on their own line.
 The text you output will be saved verbatim as the content of the file.
-Tool calls have been disabled. You MUST start your response directly with the file's new content.
+Tool calls have been disabled.
+Start your response with ```.
 
 <file_path>
 {{path}}

crates/assistant_tools/src/templates/edit_file_prompt.hbs

@@ -43,7 +43,8 @@ NEW TEXT 3 HERE
 - Always close all tags properly
 
 
-{{!-- This example is important for Gemini 2.5 --}}
+{{!-- The following example adds almost 10% pass rate for Gemini 2.5.
+Claude and gpt-4.1 don't really need it. --}}
 <example>
 <edits>