…ssions (#7394)

# Pull Request

## 📖 Description

Fixes two gaps in the `microsoft-fast-build` Rust renderer related to data access in template expressions:

1. **Array `.length` support** — `{{items.length}}` now resolves correctly. Previously, `get_nested_property` tried to parse `"length"` as a numeric array index, which always failed and returned a `MissingState` error. A special case now returns the array's length as a number.

2. **JSON literal attribute values** — Custom element attributes can now accept JSON array and object literals directly (e.g. `items='["a","b","c"]'` or `config='{"title":"Hello"}'`). Previously, these were passed through as raw strings, causing child templates that iterated or accessed nested values to fail.

### 🎫 Issues

* Helps unblock [#7390](#7390) — deep-merge fixture uses `{{users.length}}` and `{{user.orders.length}}`
* Helps unblock [#7388](#7388) — observer-map fixture uses `{{user.posts.length}}`

## 📑 Test Plan

New Rust integration tests added:

- `tests/bindings.rs` — `test_array_length`, `test_array_length_empty`, `test_array_length_nested`
- `tests/f_when.rs` — `test_when_array_length_gt_zero`, `test_when_array_length_zero`
- `tests/custom_elements.rs` — `test_custom_element_json_array_attr`, `test_custom_element_empty_array_attr`, `test_custom_element_json_object_attr`

All existing and new tests pass (`cargo test`).

## ✅ Checklist

### General

- [x] I have included a change request file using `$ npm run change`
- [x] I have added tests for my changes.
- [x] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.