fix(site): keep activity bump editable when allow_user_autostop is on (#27083) - #27142

…#27083)

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

The UI guard added in #22112 disabled the `Activity bump` field and
cleared its saved value whenever the template's `Default autostop` was
0. It did not check the "Allow users to customize autostop duration for
workspaces" (`allow_user_autostop`) setting, so templates that relied on
user-defined autostop timers had their `activity_bump_ms` silently
cleared when saving in the Coder UI.

Enable the field, preserve the value on submit, and update the helper
text when either `default_ttl_ms > 0` or `allow_user_autostop` is true.

Closes
[DEVEX-438](https://linear.app/codercom/issue/DEVEX-438/allow-user-autostop-default-autostop-disabled-causes-activity-bump-to).

> **Note:** This needs to be backported to 2.34 (ESR).

<details>
<summary>Implementation notes</summary>

### Problem

[#22112](#22112) introduced a UI
guard that:

1. Disables the `Activity bump (hours)` field when `default_ttl_ms ===
0`.
2. Sends `activity_bump_ms: undefined` on submit under the same
condition, which the backend treats as "do not update", but combined
with the disabled state users cannot re-enter a value once cleared and
the previously stored value effectively becomes orphaned.

The guard ignored `allow_user_autostop`. When that setting is enabled,
workspaces still have a scheduled stop (whatever the user configures on
their workspace), so `activity_bump_ms` is still meaningful.

### Fix

Broaden the guard to consider both signals. The field is only disabled
and the value only discarded when **both** `default_ttl_ms === 0`
**and** `allow_user_autostop === false`.

Changes:

- `TemplateScheduleForm.tsx`
- `disabled` prop now checks `!default_ttl_ms && !allow_user_autostop`.
- Submit path preserves `activity_bump_ms` when either signal is truthy.
  - Passes `allowUserAutostop` through to the helper text.
- `TTLHelperText.tsx`
- `ActivityBumpHelperText` accepts `allowUserAutostop` and only shows
the "no scheduled stop" hint when neither signal is set. Updated copy
mentions both signals.
- Tests and stories
- Existing tests explicitly uncheck `allow_user_autostop` before
asserting the guard fires (since `MockTemplate.allow_user_autostop`
defaults to `true`).
- Added coverage: guard stays off when only `allow_user_autostop` is
enabled; toggling `allow_user_autostop` re-enables the field without
touching `default_ttl_ms`.
- Added a story that verifies `activity_bump_ms` is preserved on submit
when `allow_user_autostop` is enabled and `default_ttl_ms` is 0.

</details>

(cherry picked from commit 1eea4a7)