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

coderd/mcp_http.go

@@ -9,6 +9,7 @@ import (
 	"github.com/coder/coder/v2/coderd/httpmw"
 	"github.com/coder/coder/v2/coderd/mcp"
 	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/codersdk/toolsdk"
 )
 
 type MCPToolset string
@@ -34,6 +35,7 @@ func (api *API) mcpHTTPHandler() http.Handler {
 		// Extract the original session token from the request
 		authenticatedClient := codersdk.New(api.AccessURL,
 			codersdk.WithSessionToken(httpmw.APITokenFromRequest(r)))
+		toolOpt := toolsdk.WithAgentConnFunc(api.agentProvider.AgentConn)
 		toolset := MCPToolset(r.URL.Query().Get("toolset"))
 		// Default to standard toolset if no toolset is specified.
 		if toolset == "" {
@@ -42,11 +44,11 @@ func (api *API) mcpHTTPHandler() http.Handler {
 
 		switch toolset {
 		case MCPToolsetStandard:
-			if err := mcpServer.RegisterTools(authenticatedClient); err != nil {
+			if err := mcpServer.RegisterTools(authenticatedClient, toolOpt); err != nil {
 				api.Logger.Warn(r.Context(), "failed to register MCP tools", slog.Error(err))
 			}
 		case MCPToolsetChatGPT:
-			if err := mcpServer.RegisterChatGPTTools(authenticatedClient); err != nil {
+			if err := mcpServer.RegisterChatGPTTools(authenticatedClient, toolOpt); err != nil {
 				api.Logger.Warn(r.Context(), "failed to register MCP tools", slog.Error(err))
 			}
 		default: