pw_bluetooth_proxy: Emit updates for new L2CAP channels during recovery

Conditionally emit state update notifications when registering L2CAP
channels during the offload recovery window. Channels restored from a
snapshot continue to suppress notifications to avoid duplicate records,
while newly acquired channels not present in the snapshot emit an
initial state update so the container tracks active channels.

Update L2capRecoveryTest.SnapshotUnmatchedChannelRegistration to verify
both notification suppression for restored channels and notification
emission for newly acquired Basic and Credit-Based channels during
recovery.

Bug: 536078259
Test: bazelisk test pw_bluetooth_proxy/...
Change-Id: Id34458516b9e7051e716092da7840f983177ebad
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/466425