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

[kacpersaw]

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:

<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

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)

[kacpersaw]

• feat(scaletest): extend notifications runner with smtp support #20222
• feat(cli): add mock SMTP server for testing scaletest notifications #20221 👈 (View in Graphite)
• main

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[spikecurtis]

I think you are doing this so that you can leave the port initially set to 0 and then later query the running port. It feels like a sharp edge, though, that s.SMTPAddress() will show you a 0 port until you start the server. Is there any reason why we don't want to just start the server immediately on New()?

[kacpersaw]

I'm not a fan of allocating ports in New(). IMO It should only be responsible for initialization. We could remove New() and let Start() handle init instead, since we don't do any complicated stuff in there. WDYT?

[spikecurtis]

Like:

srv := new(Server)
err = srv.Start(ctx, config)

I think that's fine.