`aggregate_channels` loses per-probe metadata

[h-mayorquin]

Coming to this thinking on:
SpikeInterface/probeinterface#420
SpikeInterface/probeinterface#425

When a probe is attached to a recording via set_probe, _set_probes stores each probe's user-level metadata in two recording-level annotations: probes_info (a list where entry i is probe i's annotations dict, with keys like name, manufacturer, description) and one probe_{i}_planar_contour annotation per probe. aggregate_channels does not propagate either of these into the combined recording. probes_info is never copied at all, so even the first child's per-probe annotations disappear. probe_{i}_planar_contour annotations are only kept when every child happens to share the exact same contour; otherwise they are all dropped. The result is that combined.get_probegroup() exposes probes with empty annotations dicts and no planar contour, regardless of what was attached to the children.

import numpy as np
from probeinterface import generate_dummy_probe
from spikeinterface.core import generate_recording, aggregate_channels

def make_recording(probe_name, manufacturer, x_shift, contact_id_prefix):
    probe = generate_dummy_probe()
    probe.move([x_shift, 0.0])
    probe.annotate(name=probe_name, manufacturer=manufacturer)
    probe.set_device_channel_indices(np.arange(probe.get_contact_count()))
    probe.set_contact_ids([f"{contact_id_prefix}{i}" for i in range(probe.get_contact_count())])
    probe.create_auto_shape()
    rec = generate_recording(num_channels=probe.get_contact_count(), durations=[1.0], set_probe=False)
    return rec.set_probe(probe)

rec_A = make_recording("probe_A", "vendor_X", x_shift=0.0,    contact_id_prefix="a")
rec_B = make_recording("probe_B", "vendor_Y", x_shift=1000.0, contact_id_prefix="b")

print("rec_A annotation keys:", rec_A.get_annotation_keys())
print("rec_A probes_info:", rec_A.get_annotation("probes_info"))

combined = aggregate_channels([rec_A, rec_B])
print("combined annotation keys:", combined.get_annotation_keys())
print("combined probes_info:", combined.get_annotation("probes_info"))
print("combined probe_0_planar_contour:", combined.get_annotation("probe_0_planar_contour"))

Expected: the aggregate carries a probes_info list with one entry per child probe and probe_{i}_planar_contour annotations for each probe.

Observed on main:

rec_A annotation keys: ['is_filtered', 'name', 'probe_0_planar_contour', 'probes_info']
rec_A probes_info: [{'manufacturer': 'vendor_X', 'name': 'probe_A'}]
combined annotation keys: ['is_filtered']
combined probes_info: None
combined probe_0_planar_contour: None