gh-160309: Fix StreamReader.read() silently dropping an incomplete final character

Lib/codecs.py

@@ -525,6 +525,15 @@ def read(self, size=-1, chars=-1, firstline=False):
             self.charbuffer += newchars
             # there was no data available
             if not newdata:
+                # The stream is exhausted.  If there are leftover bytes in
+                # bytebuffer, they form an incomplete character.  Decode them
+                # now so that the error handler is applied (strict raises,
+                # replace emits the replacement character, ignore drops them).
+                if self.bytebuffer:
+                    newchars, decodedbytes = self.decode(
+                        self.bytebuffer, self.errors)
+                    self.bytebuffer = self.bytebuffer[decodedbytes:]
+                    self.charbuffer += newchars
                 break
         if chars < 0:
             # Return everything we've got

Misc/NEWS.d/next/Library/2026-10-25-21-18-27.gh-issue-160309.wJyl0Q.rst

@@ -0,0 +1 @@
+Fix codecs.StreamReader.read() silently dropping an incomplete character at the end of the stream.