Test flake: `create instance with additional disks` in safari

We've seen this one on and off for a while.

https://github.com/oxidecomputer/console/actions/runs/13317596534/job/37195314937

    Error: Timed out 5000ms waiting for expect(locator).toHaveValue(expected)

    Locator: getByRole('dialog', { name: 'Create disk' }).getByRole('textbox', { name: 'Size (GiB)' })
    Expected string: "5"
    Received string: "10"
    Call log:
      - expect.toHaveValue with timeout 5000ms
      - waiting for getByRole('dialog', { name: 'Create disk' }).getByRole('textbox', { name: 'Size (GiB)' })
        8 × locator resolved to <input id=":r73:" value="10" type="text" autocorrect="off" autocomplete="off" spellcheck="false" inputmode="numeric" aria-labelledby=":r73:-label" aria-roledescription="Number field" class="w-full rounded border-none px-3 py-[0.6875rem] !outline-offset-1 text-sans-md text-raise bg-default placeholder:text-tertiary focus:outline-none disabled:cursor-not-allowed disabled:text-secondary disabled:bg-disabled"/>
          - unexpected value "10"


      555 |   const sizeField = createForm.getByRole('textbox', { name: 'Size (GiB)' })
      556 |   await sizeField.fill('5')
    > 557 |   await expect(sizeField).toHaveValue('5')
          |                           ^
      558 |
      559 |   await createForm.getByRole('button', { name: 'Create disk' }).click()
      560 |   await expect(createForm.getByText('Name is already in use')).toBeVisible()
        at /Users/runner/work/console/console/test/e2e/instance-create.e2e.ts:557:27

Start with test/e2e/instance-create.e2e.ts at line 557 and inspect the linked Safari Actions failure. Reproduce the create instance with additional disks flow and determine why the Size (GiB) field remains 10 after filling it with 5. Done means the Safari test reliably reaches the expected value and still verifies the duplicate-name error.