# Fix image height validation to allow the spec maximum of 400

## Problem

`Channel`/`Image` validation rejects images whose `<height>` is greater than
144, even though the RSS 2.0 specification allows heights up to 400. A
perfectly spec-compliant image is reported as invalid:

```rust
use rss::validation::Validate;
use rss::ImageBuilder;

let image = ImageBuilder::default()
    .url("https://example.com/image.png".to_string())
    .link("https://example.com/".to_string())
    .title("title".to_string())
    .height("200".to_string()) // valid per the RSS spec
    .build();

assert!(image.validate().is_err()); // <- wrongly errors today
```

## Root cause

`src/validation.rs`, in `impl Validate for Image`, validated `height` against
the same range used for `width`:

```rust
let height = height.parse::<i64>()?;
validate!(
    (0..=144).contains(&height),
    "Image height is not between 0 and 144"
);
```

The RSS 2.0 specification (<https://www.rssboard.org/rss-specification>,
`<image>` sub-element) states:

- width: "Maximum value for width is 144, default value is 88."
- height: "Maximum value for height is 400, default value is 31."

So 144 is correct for `width`, but `height` must allow up to 400. The bound
(and the error message) were copied from the width check.

## Fix

Validate `height` against `0..=400` and correct the error message:

```rust
let height = height.parse::<i64>()?;
validate!(
    (0..=400).contains(&height),
    "Image height is not between 0 and 400"
);
```

`width` is unchanged and keeps its specification maximum of 144.

## Test

Added `tests/validation.rs` (gated on the `validation` feature):

- `image_height_allows_up_to_400`: heights 31, 200 and 400 validate OK.
- `image_height_above_400_is_invalid`: height 401 is rejected.
- `image_width_max_is_still_144`: width 144 is OK, 145 is rejected.

## Gate

- `cargo test --all-features` -> ok (all suites pass, exit 0)
- `cargo build --all-features` -> ok (exit 0)
- `cargo fmt --check` -> ok (exit 0)

## Counterfactual

Reverting the bound back to `0..=144` makes the new test fail:

```
thread 'image_height_allows_up_to_400' panicked at tests/validation.rs:28:5:
assertion failed: image_with_height("200").validate().is_ok()
```

## Flags for the account owner

`cargo clippy --all-targets --all-features -- -D warnings` currently fails, but
this is pre-existing and unrelated to this change: on an unmodified checkout the
same lint (`clippy::get_first`, i.e. "accessing first element with `.get(0)`",
emitted by a newer Clippy than the tests were written against) already produces
these errors in `tests/read.rs`. The files touched here are clippy-clean.