Lay the groundwork for a Rust-based eval (#28488)

Also, we moved the logic for driving the agentic loop into `Thread` so
that we don't have to re-implement it.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/eval/examples/find_and_replace_diff_card/base.toml

@@ -0,0 +1,2 @@
+path = "../zed_worktree"
+revision = "38fcadf9481d018543c65f36ac3bafeba190179b"

crates/eval/examples/find_and_replace_diff_card/prompt.md

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+Look at the `find_replace_file_tool.rs`. I want to implement a card for it. The card should be a brand new `Entity` with a `Render` implementation.
+
+The card should show a diff. It should be a beautifully presented diff. The card "box" should look like what we show for markdown codeblocks (look at `MarkdownElement`). I want to see a red background for lines that were deleted and a green background for lines that were added. We should have a div per diff line.