M2.6: the JBIG2 cap's doc and test say what happens

The decoder has the same 2^28 cap as decode_jbig2, and can refuse a page first, so the doc says either
error can come. The page at the cap decodes, which the test now asserts, rather than failing later.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AmsdeUCdTaD72jkDw2pJKY

crates/filters/src/jbig2/decode.rs

@@ -20,7 +20,8 @@ pub const MAX_JBIG2_PIXELS: u64 = 1 << 28;
 ///
 /// Errors: `data` or `globals` that don't decode, or a page of another size
 /// ([`FilterError::Corrupt`]); a page past `max` ([`FilterError::LimitExceeded`]) or
-/// [`MAX_JBIG2_PIXELS`] ([`FilterError::Unsupported`]).
+/// [`MAX_JBIG2_PIXELS`] ([`FilterError::Unsupported`], or `Corrupt` from the decoder, which has the
+/// same cap and can refuse the page first).
 pub fn decode_jbig2(data: &[u8], globals: Option<&[u8]>, columns: u32, rows: u32, max: usize) -> Result<Vec<u8>, FilterError> {
     if rows == 0 {
         return Ok(Vec::new());
@@ -137,11 +138,11 @@ mod tests {
         let page = 26 * h as usize;
         assert!(matches!(decode_jbig2(&data, None, w, h, page - 1), Err(FilterError::LimitExceeded(m)) if m == page - 1), "a byte short");
         assert_eq!(decode_jbig2(&data, None, w, h, page).unwrap().len(), page, "the page within max");
-        // 16,384 × 16,384 is 2^28 pixels, and fails only later; a row more is past the cap, here or
-        // in the decoder, which can refuse it first.
+        // 16,384 × 16,384 is 2^28 pixels, which decodes; a row more is past the cap, here or in the
+        // decoder, which has the same one and can refuse it first.
         let large = |rows| decode_jbig2(&page_of(&data, 16_384, rows), None, 16_384, rows, usize::MAX);
         assert!(matches!(large(16_385), Err(FilterError::Unsupported(_) | FilterError::Corrupt { .. })), "{:?}", large(16_385));
-        assert!(!matches!(large(16_384), Err(FilterError::Unsupported(_))), "{:?}", large(16_384));
+        assert!(large(16_384).is_ok(), "{:?}", large(16_384).map(|page| page.len()));
 
         // An extension segment (ISO/IEC 14492 7.4.14), which a decoder skips: its 11-byte header
         // gives the length of the data after it.