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

Cargo.lock

@@ -683,6 +683,7 @@ dependencies = [
  "language_models",
  "log",
  "markdown",
+ "once_cell",
  "open",
  "paths",
  "portable-pty",