Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This is a clean, minimal sample driver that demonstrates the serdev abstraction.

**1. Probe error handling — discards actual baudrate on error (lines 220–229):**

```rust
if sdev
    .set_baudrate(
        dev.fwnode()
            .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
            .unwrap_or(115200),
    )
    .is_err()
{
    return Err(EINVAL);
}
```

`set_baudrate` returns `Result<(), u32>` where the `Err(u32)` contains the actual baudrate that was set. The sample discards this. Fine for a sample, but a real driver might want to log or use the actual rate.

**2. `receive` callback echoes data — good illustrative choice:**

```rust
fn receive<'bound>(
    sdev: &'bound serdev::Device<Bound>,
    _this: Pin<&Self>,
    data: &[u8],
) -> usize {
    let _ = sdev.write_all(data, serdev::Timeout::Max);
    data.len()
}
```

The echo pattern is a good sample. Ignoring the `write_all` result with `let _ = ...` is acceptable here since there's no meaningful recovery in a sample echo driver.

**3. Kconfig correctly selects `RUST_SERIAL_DEV_BUS_ABSTRACTIONS`.**

No issues.

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