feat: add TagV2 BoundaryMessage envelope protocol (#22520)

Extend the wire protocol for the boundary <-> agent unix socket with
a message envelope.

The envelope creates a boundary <-> agent data path that is separate
from the agent <-> coderd path. This lets boundary send operational
metadata (drop counts, configuration like jail type, capabilities)
that the agent can act on locally (e.g. Prometheus metrics) or use
to enrich outbound requests, without polluting the coderd-facing proto
with fields coderd never consumes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agent/boundarylogproxy/codec/codec_test.go

@@ -89,7 +89,7 @@ func TestReadFrameInvalidTag(t *testing.T) {
 	// reading the invalid tag.
 	const (
 		dataLength uint32 = 10
-		bogusTag   uint32 = 2
+		bogusTag   uint32 = 222
 	)
 	header := bogusTag<<codec.DataLength | dataLength
 	data := make([]byte, 4)
@@ -139,7 +139,7 @@ func TestWriteFrameInvalidTag(t *testing.T) {
 
 	var buf bytes.Buffer
 	data := make([]byte, 1)
-	const bogusTag = 2
+	const bogusTag = 222
 	err := codec.WriteFrame(&buf, codec.Tag(bogusTag), data)
 	require.ErrorIs(t, err, codec.ErrUnsupportedTag)
 }