CBL-8468 : Fix incoming BLE L2CAP socket double-free on connection teardown

[pasin]

The btAttached() (incoming) now calls c4socket_retain, matching btOpen(), to balance the c4socket_release in NativeC4Socket_closed. Without it the incoming socket was double-freed on teardown (SIGABRT, invalid refCount -6666666).

[github-actions]

This is a release branch and commits are restricted.

Please confirm this PR is one of the following:

• A response to a customer ask
• A change per our security policy
• A non-functional change (i.e. changes needed for building an older version)
• A change that has been granted an exception (please comment)

[pasin]

Fix the issue while testing Bluetooth functionality for the release.

[Copilot]

Pull request overview

Fixes a lifecycle mismatch for incoming BLE L2CAP sockets by retaining the C4Socket in btAttached() to match the existing outgoing path in btOpen(), preventing a double-free during teardown when Java later calls NativeC4Socket_closed (which releases the socket).

Changes:

• Add an explanatory comment and keep the existing c4socket_retain() in btOpen() to document the retain/release pairing.
• Add c4socket_retain() in btAttached() (incoming socket path) so it’s balanced by the existing c4socket_release() in NativeC4Socket_closed.

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