fix(msgpack): use fixstr encoding for strings of length 20-31 #36737

Problem:
MessagePack fixstr format supports string lengths 0-31, but mpack_str()
only used fixstr for lengths < 20. Strings of 20-31 bytes were
incorrectly encoded as str8 (2-byte header) instead of fixstr (1-byte
header).

Solution:
Change the condition from `len < 20` to `len < 32` to match the
MessagePack specification.

This fix affects message timing which exposed a pre-existing race
condition in the channels_spec PTY test. The test now uses a helper
function to accumulate partial PTY reads, making it more robust.

Fixes #32784

test/functional/core/channels_spec.lua

@@ -196,6 +196,22 @@ describe('channels', function()
     end
   end
 
+  -- Helper to accumulate PTY stdout data until expected string is received.
+  -- PTY reads are non-atomic and may deliver data in chunks.
+  local function expect_stdout(id, expected)
+    local accumulated = ''
+    while #accumulated < #expected do
+      local msg = next_msg()
+      eq('notification', msg[1])
+      eq('stdout', msg[2])
+      eq(id, msg[3][1])
+      for _, chunk in ipairs(msg[3][2]) do
+        accumulated = accumulated .. chunk
+      end
+    end
+    eq(expected, accumulated)
+  end
+
   it('can use stdio channel with pty', function()
     skip(is_os('win'))
     source([[
@@ -229,7 +245,7 @@ describe('channels', function()
     expect_twoline(id, 'stdout', 'Blobs!\r', "[1, ['Blobs!', ''], 'stdin']")
 
     command("call chansend(id, 'neovan')")
-    eq({ 'notification', 'stdout', { id, { 'neovan' } } }, next_msg())
+    expect_stdout(id, 'neovan')
     command("call chansend(id, '\127\127im\n')")
     expect_twoline(id, 'stdout', '\b \b\b \bim\r', "[1, ['neovim', ''], 'stdin']")