fix!: deprecate login_type=none, convert existing users to password login (#26851)

> 🤖 This PR was modified by Coder Agents on behalf of Jake Howell.

Deprecates `login_type=none` (legacy passwordless machine users) in
favour of premium **service accounts**, and migrates existing accounts
off the deprecated path while preserving their identity. Resolves
[DEVEX-226].

## What this does

- **Creation is gated** — `POST /users` and `coder users create` reject
`login_type=none` (and the deprecated `--disable-login`) unless a
service account is requested.
- **Existing users are converted** — migration
`000554_legacy_none_login_to_password` rewrites legacy non-system,
non–service-account `login_type='none'` accounts to
`login_type='password'`. Email addresses are **preserved** and existing
API tokens remain valid. Admins can set a password if interactive login
is desired.

## Why convert to `password` and not `is_service_account`?

Migration `000433_add_is_service_account_to_users` adds two CHECK
constraints:

- `users_email_not_empty`: `(is_service_account = true) = (email = '')`
- `users_service_account_login_type`: `is_service_account = false OR
login_type = 'none'`

Turning a real, email-bearing `login_type=none` user into a service
account would require **blanking their email**. Converting to `password`
instead preserves the account and its email.

> ⚠️ **Breaking / one-way.** The `down` migration cannot restore which
users originally had `login_type='none'`.


Decision log

- **Goal:** move existing `login_type=none` users off the deprecated
path while preserving their identity/email.
- **Constraint discovered:** the `is_service_account` CHECK constraints
(migration `000433`) make a literal `none → service account` conversion
require blanking emails, so this PR converts to `password` instead to
keep emails intact.
- **Implementation:** creation-gating in `cli/usercreate.go` and
`coderd/users.go`, matching test updates, plus the
`000554_legacy_none_login_to_password.{up,down}.sql` migration.
- **CI fix:** the branch was behind `main` and its migration originally
numbered `000534`, which collided with main's
`000534_drop_chat_model_configs_provider`. Merged `main` and renumbered
to `000554` (next free after main's `000553`). `make gen` produces no
drift (the migration is data-only).



> The service-account conversion alternative (#27182, which blanked
emails) was closed in favour of this password-preserving approach.
>
> Docs follow-up: #27333.

[DEVEX-226]: https://linear.app/issue/DEVEX-226

---------

Co-authored-by: Sushant P <zenithwolf1000@users.noreply.github.com>

coderd/users.go

@@ -488,6 +488,13 @@ func (api *API) postUser(rw http.ResponseWriter, r *http.Request) {
 		req.UserLoginType = codersdk.LoginTypePassword
 	}
 
+	if !req.ServiceAccount && req.UserLoginType == codersdk.LoginTypeNone {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Login type 'none' requires a service account.",
+		})
+		return
+	}
+
 	if req.UserLoginType != codersdk.LoginTypePassword && req.Password != "" {
 		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
 			Message: fmt.Sprintf("Password cannot be set for non-password (%q) authentication.", req.UserLoginType),