FIX: Keep extras when combining Annotations

[sametSeckiiin]

Fixes #14022 What does this implement/fix?

When combining annotations using _combine_annotations (e.g., during raw concatenations or appending), the extras list associated with the annotations was being dropped. While onset, duration, and description arrays were properly concatenated, the new Annotations object was constructed without passing the combined extras keyword argument.

This PR fixes the issue by:

• Merging the extras lists from the input annotation objects (list(one.extras) + list(two.extras)).
• Passing the merged extras list to the returned Annotations constructor so that metadata like response times or stimulus details are preserved after concatenation.

Additional information

Here is a minimal script to verify the fix:
`import mne
a1 = mne.Annotations([1], [1], ['stim'], extras=[{'res': 1}])
a2 = mne.Annotations([2], [2], ['stim'], extras=[{'res': 2}])
print((a1 + a2).extras)

Output is now correctly: [{'res': 1}, {'res': 2}] instead of [{}]

[welcome]

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