fix(validation): allow image height up to spec maximum of 400

Image height was validated against 0..=144, the same bound as width, but the
RSS 2.0 specification allows heights up to 400 (width max is 144). Spec-valid
images with a height between 145 and 400 were wrongly reported as invalid.
Validate height against 0..=400 and fix the error message.

PR.md

@@ -0,0 +1,88 @@
+# 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.

src/validation.rs

@@ -222,8 +222,8 @@ impl Validate for Image {
         if let Some(height) = self.height() {
             let height = height.parse::<i64>()?;
             validate!(
-                (0..=144).contains(&height),
-                "Image height is not between 0 and 144"
+                (0..=400).contains(&height),
+                "Image height is not between 0 and 400"
             );
         }
 

tests/validation.rs

@@ -0,0 +1,42 @@
+#![cfg(feature = "validation")]
+
+use rss::validation::Validate;
+use rss::ImageBuilder;
+
+fn image_with_height(height: &str) -> rss::Image {
+    ImageBuilder::default()
+        .url("https://example.com/image.png".to_string())
+        .link("https://example.com/".to_string())
+        .title("title".to_string())
+        .height(height.to_string())
+        .build()
+}
+
+fn image_with_width(width: &str) -> rss::Image {
+    ImageBuilder::default()
+        .url("https://example.com/image.png".to_string())
+        .link("https://example.com/".to_string())
+        .title("title".to_string())
+        .width(width.to_string())
+        .build()
+}
+
+#[test]
+fn image_height_allows_up_to_400() {
+    // The RSS 2.0 specification sets the maximum image height to 400.
+    assert!(image_with_height("31").validate().is_ok());
+    assert!(image_with_height("200").validate().is_ok());
+    assert!(image_with_height("400").validate().is_ok());
+}
+
+#[test]
+fn image_height_above_400_is_invalid() {
+    assert!(image_with_height("401").validate().is_err());
+}
+
+#[test]
+fn image_width_max_is_still_144() {
+    // Width keeps its specification maximum of 144.
+    assert!(image_with_width("144").validate().is_ok());
+    assert!(image_with_width("145").validate().is_err());
+}