Fix incorrect Annotation::fetchAll() return structure for single Examples attribute

[xEdelweiss]

I had an issue similar to #6726, caused by how Annotation::fetchAll() handles the single #[Examples] attribute.

Here are comparison of fetchAll() return structures:

# Multiple Example attributes:
array (
  0 => array (
    0 => 'example 1',
  ),
  1 => array (
    0 => 'example 2',
  ),
)

# ❌ Single Example attribute (actual):
array (
  0 => 'example 1', 
)

# ✔️ Single Example attribute (expected):
array (
  0 => array (
    0 => 'example 1',
  ),
)

This PR includes a fix and unit-tests update for this issue. It also has a test for the #[Given] attribute to show that the changes do not break it.