fix: reuse shared tailnet for coderd-hosted MCP workspace tools (#24460)

## Problem

Coderd can expose an MCP server at `/api/experimental/mcp/http` (we have
this enabled on dogfood). Its workspace tools dialed agents through a
per-call client-side tailnet stack. Every tool call re-created a
WireGuard device, netstack, magicsock + UDP sockets, DERP connection,
coordinator websocket, and their goroutines — in a process that already
runs a long-lived shared tailnet. The duplicate stacks drove up resource
usage under load.

## Fix

Route this server's tool calls through the existing shared tailnet, so
none of those transports are reconstructed per call. Closing an
`AgentConn` now releases a tunnel reference instead of tearing down a
transport.

## Potential follow-up

`coder exp mcp server` still builds a fresh tailnet per call. It pays
per-call latency and causes coordinator/DERP churn. A shared CLI tailnet
is more involved — unlike coderd, the CLI has no existing shared tailnet
to reuse, so it would need a new long-lived client-side tailnet with
reconnect, sleep/wake, and idle-destination handling. There's less
motivation to optimize this, given the client-side MCP does not compete
for resources with coderd.

Closes CODAGT-199

> Generated by mux, but reviewed by a human

codersdk/workspacesdk/agentconn.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/binary"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -43,6 +44,40 @@ func NewAgentConn(conn *tailnet.Conn, opts AgentConnOptions) AgentConn {
 	}
 }
 
+// WrapAgentConn returns an AgentConn that delegates every operation to conn and
+// applies closeFunc exactly once when the logical session is closed.
+//
+// If conn is nil, any provided closeFunc is invoked immediately so logical
+// session cleanup is not silently dropped.
+func WrapAgentConn(conn AgentConn, closeFunc func() error) AgentConn {
+	if conn == nil {
+		if closeFunc != nil {
+			_ = closeFunc()
+		}
+		return nil
+	}
+	if closeFunc == nil {
+		closeFunc = func() error { return nil }
+	}
+	return &wrappedAgentConn{AgentConn: conn, closeFunc: closeFunc}
+}
+
+type wrappedAgentConn struct {
+	AgentConn
+	closeFunc func() error
+	closeOnce sync.Once
+	closeErr  error
+}
+
+func (c *wrappedAgentConn) Close() error {
+	c.closeOnce.Do(func() {
+		// Close the underlying connection before releasing the logical session so
+		// the lease remains held until teardown is complete.
+		c.closeErr = errors.Join(c.AgentConn.Close(), c.closeFunc())
+	})
+	return c.closeErr
+}
+
 const (
 	// CoderChatIDHeader is the HTTP header containing the current
 	// chat ID. Set by coderd on agentconn requests originating