M2.6: autotagging's reason for content it can't decode reads as on main

Since autotagging reads page content through DecodeBudget::decode_strict, the reason it gave for
content that doesn't decode carried that error's own text, so it said twice that the data
couldn't be decoded: "content cannot be decoded strictly: stream data could not be decoded:
FlateDecode: …". It now gives the decoder's reason right after, as main does: "content cannot
be decoded strictly: FlateDecode: …".

Test: autotag_text_says_why_it_cannot_read_a_page now checks the decoder's reason follows the
prefix.

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

crates/a11y/src/autotag.rs

@@ -66,7 +66,13 @@ fn content_bytes(doc: &Document, page: &pdfcraft_model::Page, index: usize) -> R
             if budget.remaining() == 0 {
                 unsupported(index, "decoding the content takes more than 16 MiB")
             } else {
-                unsupported(index, format!("content cannot be decoded strictly: {e}"))
+                // The decoder's own reason: `CosError::Filter`'s text would say again that the data
+                // couldn't be decoded.
+                let why = match e {
+                    pdfcraft_cos::CosError::Filter(why) => why,
+                    other => other.to_string(),
+                };
+                unsupported(index, format!("content cannot be decoded strictly: {why}"))
             }
         })?;
         all.extend_from_slice(&decoded);

crates/a11y/src/tests.rs

@@ -113,7 +113,7 @@ fn autotag_text_reads_content_with_parameters_given_by_reference() {
 }
 
 /// Autotagging refuses a page whose content it can't read whole, and says why: content past
-/// 16 MiB, or content that doesn't decode strictly.
+/// 16 MiB, or content that doesn't decode strictly, with the decoder's reason right after.
 #[test]
 fn autotag_text_says_why_it_cannot_read_a_page() {
     use pdfcraft_cos::{Dict, ObjRef, Stream};
@@ -138,7 +138,7 @@ fn autotag_text_says_why_it_cannot_read_a_page() {
     let mut corrupt = Stream::from_raw(Dict::new(), b"not zlib data".to_vec());
     corrupt.dict.set(b"Filter".to_vec(), Object::name("FlateDecode"));
     let why = reason(corrupt);
-    assert!(why.starts_with("content cannot be decoded strictly"), "{why}");
+    assert!(why.starts_with("content cannot be decoded strictly: FlateDecode: "), "{why}");
 }
 
 #[test]