feat(cli): add mock SMTP server for testing scaletest notifications (#20221)

This PR adds a fake SMTP server for scale testing. It collects emails sent during tests, which you can then check using the HTTP API.

#### Changes
- Added mock SMTP server  
- Added `coder scaletest smtp` CLI command  
- Implemented HTTP API endpoints to retrieve messages by email  
- Added auto-purge to prevent memory issues  

#### HTTP API Endpoints
- `GET /messages?email=<email>` – Get messages sent to an email address  
- `POST /purge` – Clear all messages from memory  

The HTTP API parses raw email messages to extract the **date**, **subject**, and **notification ID**.

Notification IDs are sent in emails like this:
```html
<p>
  <a href="http://127.0.0.1:3000/settings/notifications?disabled=4e19c0ac-94e1-4532-9515-d1801aa283b2"
     style="color: #2563eb; text-decoration: none;">
    Stop receiving emails like this
  </a>
</p>
```

#### CLI
```bash
coder scaletest smtp --host localhost --port 33199 --api-port 8080 --purge-at-count 1000
```

**Flags:**
- `--host`: Host for the mock SMTP and API server (default: localhost)  
- `--port`: Port for the mock SMTP server (random if not specified)  
- `--api-port`: Port for the HTTP API server (random if not specified)  
- `--purge-at-count`: Max number of messages before auto-purging (default: 100000)

cli/exp_scaletest.go

@@ -66,6 +66,7 @@ func (r *RootCmd) scaletestCmd() *serpent.Command {
 			r.scaletestWorkspaceTraffic(),
 			r.scaletestAutostart(),
 			r.scaletestNotifications(),
+			r.scaletestSMTP(),
 		},
 	}
 

cli/exp_scaletest_smtp.go

@@ -0,0 +1,112 @@
+//go:build !slim
+
+package cli
+
+import (
+	"fmt"
+	"os/signal"
+	"time"
+
+	"golang.org/x/xerrors"
+
+	"cdr.dev/slog"
+	"cdr.dev/slog/sloggers/sloghuman"
+	"github.com/coder/coder/v2/scaletest/smtpmock"
+	"github.com/coder/serpent"
+)
+
+func (*RootCmd) scaletestSMTP() *serpent.Command {
+	var (
+		hostAddress  string
+		smtpPort     int64
+		apiPort      int64
+		purgeAtCount int64
+	)
+	cmd := &serpent.Command{
+		Use:   "smtp",
+		Short: "Start a mock SMTP server for testing",
+		Long: `Start a mock SMTP server with an HTTP API server that can be used to purge
+messages and get messages by email.`,
+		Handler: func(inv *serpent.Invocation) error {
+			ctx := inv.Context()
+			notifyCtx, stop := signal.NotifyContext(ctx, StopSignals...)
+			defer stop()
+			ctx = notifyCtx
+
+			logger := slog.Make(sloghuman.Sink(inv.Stderr)).Leveled(slog.LevelInfo)
+			config := smtpmock.Config{
+				HostAddress: hostAddress,
+				SMTPPort:    int(smtpPort),
+				APIPort:     int(apiPort),
+				Logger:      logger,
+			}
+			srv := new(smtpmock.Server)
+
+			if err := srv.Start(ctx, config); err != nil {
+				return xerrors.Errorf("start mock SMTP server: %w", err)
+			}
+			defer func() {
+				_ = srv.Stop()
+			}()
+
+			_, _ = fmt.Fprintf(inv.Stdout, "Mock SMTP server started on %s\n", srv.SMTPAddress())
+			_, _ = fmt.Fprintf(inv.Stdout, "HTTP API server started on %s\n", srv.APIAddress())
+			if purgeAtCount > 0 {
+				_, _ = fmt.Fprintf(inv.Stdout, "  Auto-purge when message count reaches %d\n", purgeAtCount)
+			}
+
+			ticker := time.NewTicker(10 * time.Second)
+			defer ticker.Stop()
+
+			for {
+				select {
+				case <-ctx.Done():
+					_, _ = fmt.Fprintf(inv.Stdout, "\nTotal messages received since last purge: %d\n", srv.MessageCount())
+					return nil
+				case <-ticker.C:
+					count := srv.MessageCount()
+					if count > 0 {
+						_, _ = fmt.Fprintf(inv.Stdout, "Messages received: %d\n", count)
+					}
+
+					if purgeAtCount > 0 && int64(count) >= purgeAtCount {
+						_, _ = fmt.Fprintf(inv.Stdout, "Message count (%d) reached threshold (%d). Purging...\n", count, purgeAtCount)
+						srv.Purge()
+						continue
+					}
+				}
+			}
+		},
+	}
+
+	cmd.Options = []serpent.Option{
+		{
+			Flag:        "host-address",
+			Env:         "CODER_SCALETEST_SMTP_HOST_ADDRESS",
+			Default:     "localhost",
+			Description: "Host address to bind the mock SMTP and API servers.",
+			Value:       serpent.StringOf(&hostAddress),
+		},
+		{
+			Flag:        "smtp-port",
+			Env:         "CODER_SCALETEST_SMTP_PORT",
+			Description: "Port for the mock SMTP server. Uses a random port if not specified.",
+			Value:       serpent.Int64Of(&smtpPort),
+		},
+		{
+			Flag:        "api-port",
+			Env:         "CODER_SCALETEST_SMTP_API_PORT",
+			Description: "Port for the HTTP API server. Uses a random port if not specified.",
+			Value:       serpent.Int64Of(&apiPort),
+		},
+		{
+			Flag:        "purge-at-count",
+			Env:         "CODER_SCALETEST_SMTP_PURGE_AT_COUNT",
+			Default:     "100000",
+			Description: "Maximum number of messages to keep before auto-purging. Set to 0 to disable.",
+			Value:       serpent.Int64Of(&purgeAtCount),
+		},
+	}
+
+	return cmd
+}