eval: Add HTML overview for evaluation runs (#29413)

This update generates a single self-contained .html file that shows an
overview of evaluation threads in the browser. It's useful for:

- Quickly reviewing results
- Sharing evaluation runs
- Debugging
- Comparing models (TBD)

Features:

- Export thread JSON from the UI
- Keyboard navigation (j/k or Ctrl + ←/→)
- Toggle between compact and full views

Generating the overview:

- `cargo run -p eval` will write this file in the run dir's root.
- Or you can call `cargo run -p eval --bin explorer` to generate it
without running evals.


Screenshot:

![image](https://github.com/user-attachments/assets/4ead71f6-da08-48ea-8fcb-2148d2e4b4db)


Release Notes:

- N/A

crates/eval/README.md

@@ -5,3 +5,21 @@ This eval assumes the working directory is the root of the repository. Run it wi
 ```sh
 cargo run -p eval
 ```
+
+## Explorer Tool
+
+The explorer tool generates a self-contained HTML view from one or more thread
+JSON file. It provides a visual interface to explore the agent thread, including
+tool calls and results. See [./docs/explorer.md](./docs/explorer.md) for more details.
+
+### Usage
+
+```sh
+cargo run -p eval --bin explorer -- --input <path-to-json-files> --output <output-html-path>
+```
+
+Example:
+
+```sh
+cargo run -p eval --bin explorer -- --input ./runs/2025-04-23_15-53-30/fastmcp_bugifx/*/last.messages.json --output /tmp/explorer.html
+```