fix(attachments): address PR review feedback (Copilot + code-quality)

- TextDecoder in the text-preview stream now flushes on completion/cancel, so a multibyte UTF-8 char split across a chunk boundary isn't dropped (Copilot).

- send() clears self._wire_part_cache in a finally so the per-send memo (which can hold large rasterized PDF page-images) is released at send end instead of retained on an idle session until the next send (Copilot + fix-review).

- Make the implicit byte-string concatenation in _minimal_pdf explicit (+) in test_pdf.py and test_thumbnails.py so it can't read as a missing comma (CodeQL / github-code-quality).

tests/test_pdf.py

@@ -12,7 +12,7 @@ def _minimal_pdf(text: str = "Hello PDF") -> bytes:
         b"<</Type/Catalog/Pages 2 0 R>>",
         b"<</Type/Pages/Kids[3 0 R]/Count 1>>",
         b"<</Type/Page/Parent 2 0 R/MediaBox[0 0 300 144]"
-        b"/Contents 4 0 R/Resources<</Font<</F1 5 0 R>>>>>>",
+        + b"/Contents 4 0 R/Resources<</Font<</F1 5 0 R>>>>>>",
         b"<</Length %d>>\nstream\n%s\nendstream" % (len(stream), stream),
         b"<</Type/Font/Subtype/Type1/BaseFont/Helvetica>>",
     ]

tests/test_thumbnails.py

@@ -20,7 +20,7 @@ def _minimal_pdf(text: str = "Hi") -> bytes:
         b"<</Type/Catalog/Pages 2 0 R>>",
         b"<</Type/Pages/Kids[3 0 R]/Count 1>>",
         b"<</Type/Page/Parent 2 0 R/MediaBox[0 0 300 144]"
-        b"/Contents 4 0 R/Resources<</Font<</F1 5 0 R>>>>>>",
+        + b"/Contents 4 0 R/Resources<</Font<</F1 5 0 R>>>>>>",
         b"<</Length %d>>\nstream\n%s\nendstream" % (len(stream), stream),
         b"<</Type/Font/Subtype/Type1/BaseFont/Helvetica>>",
     ]

turnstone/core/session.py

@@ -2969,9 +2969,9 @@ class ChatSession:
         # re-rasterized / a blob re-base64'd once per round-trip.  Key on
         # (id, caps-signature): the same stored blob materializes differently per
         # capability set, and a fallback to a different-caps model can resolve
-        # within one send.  The cache is refreshed per send (set in send()); the
-        # wire resolver runs only during a send, so a stale value between sends is
-        # never read.  A None cache disables memoization (the original behavior).
+        # within one send.  Set in send() and cleared in its finally, so it is
+        # None outside a send; the wire resolver runs only during a send.  A None
+        # cache disables memoization (the original behavior).
         cache = self._wire_part_cache
         caps_sig = (caps.supports_pdf, caps.supports_vision, caps.supports_audio_input)
         out: dict[str, Any] = {}
@@ -4600,6 +4600,12 @@ class ChatSession:
             self._drain_pending_advisories()
             self._record_fatal_error(exc)
             raise
+        finally:
+            # Release the per-send wire-part memo (it can hold large rasterized
+            # PDF page-images) so it is GC'd at send end rather than retained on
+            # an idle session until the next send.  Restores the "None outside a
+            # send" invariant on every exit (success, cancel, or error).
+            self._wire_part_cache = None
 
     def _drain_pending_advisories(self) -> None:
         """Drop every pending nudge regardless of channel.

turnstone/shared_static/composer_attachments.js

@@ -139,6 +139,7 @@ export function buildAttachmentPreview(opts) {
             if (res && res.value)
               acc += dec.decode(res.value, { stream: true });
             if (res.done || acc.length >= 240) {
+              acc += dec.decode(); // flush bytes buffered across a chunk boundary
               try {
                 reader.cancel();
               } catch (e) {