Only show message to stop instance when instance has been selected

[charliepark]

A small logic update to check for the presence of a selected instance before showing the "plz stop instance" message box. Also updates the copy from "An instance" to "This instance"

Closes #2943

No selected instance:

A not-stopped instance has been selected:

O ho! A stopped instance has been selected:

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Project	Deployment	Preview	Updated (UTC)
console	Ready	Preview	Nov 4, 2025 5:22am

[charliepark]

I think adding a test here is overkill, but on the off chance we want to add one, this one is ready to go.

test('add instance modal shows notice only for non-stopped instances', async ({ page }) => {
  await page.goto('/projects/mock-project/affinity/oil-water')

  const addInstanceButton = page.getByRole('button', { name: 'Add instance' })
  const addInstanceModal = page.getByRole('dialog', { name: 'Add instance to group' })
  const instanceCombobox = page.getByRole('combobox', { name: 'Instance' })
  const notice = page.getByText('An instance must be stopped to add it to a group')
  const modalAddButton = page.getByRole('button', { name: 'Add to group' })

  // Open modal - notice should not be visible when no instance is selected
  await addInstanceButton.click()
  await expect(addInstanceModal).toBeVisible()
  await expect(notice).toBeHidden()

  // Select a running instance - notice should appear and button should be disabled
  await page.getByRole('option', { name: 'db1' }).click()
  await expect(instanceCombobox).toHaveValue('db1')
  await expect(notice).toBeVisible()
  await expect(modalAddButton).toBeDisabled()

  // Close modal and stop db1
  await page.getByRole('button', { name: 'Cancel' }).click()
  await expect(addInstanceModal).toBeHidden()
  await page.getByRole('link', { name: 'Instances' }).click()
  await clickRowAction(page, 'db1', 'Stop')
  await page.getByRole('button', { name: 'Confirm' }).click()
  await expectRowVisible(page.getByRole('table'), {
    name: 'db1',
    state: expect.stringContaining('stopped'),
  })

  // Go back to the anti-affinity group
  await page.getByRole('link', { name: 'Affinity' }).click()
  await page.getByRole('link', { name: 'oil-water' }).click()

  // Open modal again and select the now-stopped instance
  await addInstanceButton.click()
  await expect(addInstanceModal).toBeVisible()
  await page.getByRole('option', { name: 'db1' }).click()
  await expect(instanceCombobox).toHaveValue('db1')

  // Notice should NOT appear and button should be enabled for stopped instance
  await expect(notice).toBeHidden()
  await expect(modalAddButton).toBeEnabled()

  // Close modal
  await page.getByRole('button', { name: 'Cancel' }).click()
  await expect(addInstanceModal).toBeHidden()
})

[david-crespo]

This looks good. It's kind of a shame that we tell the user that an instance needs to be stopped, but we don't give any way of actually doing that. As the proposed test shows, you have to close the modal and nav to instances to stop it. Ben has always talked about having more inline actions for these cases.

If you want to add a test, that test is too much — I'd lean toward adding 5 lines to an existing test, picking one instance that is already stopped and one that isn't.