connector-contract-v3: drop the unreachable short-string arms of the str header width

Mutation survivors on 2101602: the worst-case payload is the base64 of an
envelope holding two 256-byte ids, so it never fits a one- or two-byte str
header.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/model-connector/src/envelope.rs

@@ -226,14 +226,11 @@ fn bin_header_bytes(len: usize) -> usize {
     }
 }
 
-/// MessagePack `str` header width for a string of `len` bytes.
+/// MessagePack `str` header width for the base64 payload. The payload always
+/// exceeds 255 bytes: the envelope it encodes holds two ids of at least
+/// `MAX_IDENTITY_BYTES` in the worst case this bound measures.
 fn str_header_bytes(len: usize) -> usize {
-    match len {
-        0..=31 => 1,
-        32..=255 => 2,
-        256..=65_535 => 3,
-        _ => 5,
-    }
+    if len <= 65_535 { 3 } else { 5 }
 }
 
 /// The largest payload bound whose worst-case frame fits `max_frame_bytes`: