[Dikshant-Neupane]

Closes #1564

When a comm message arrives while an async cell is running, the comm is dispatched concurrently while the cell holds the shell lock, and pre_execute/post_execute fire for the comm callback mid-cell. Any output a post_execute hook publishes is then parented to the comm_msg instead of the cell's execute_request, so frontends drop it, and the hook's queue is drained before the cell's own post_execute fires.

The sync-cell case was already fixed by #1565; this covers the remaining async-cell case, with and without a subshell.

Fix: a context variable is set in the shell_main concurrent-dispatch branch only, and ipykernel's BaseComm.handle_msg override skips the two events when it is set. Comms handled while no cell is running keep the previous behaviour. The approach follows the discussion in the issue.

Repro (issue's reproducer, on main f87d0e0):

• Sync cell + subshell: passed before and after (fixed by Allow async cell to process Comm messages #1565).
• Async cell + subshell: failed before (display parented to comm_msg, post_execute at about 1s mid-cell), passes after.
• Async cell, no subshell: failed before, passes after.

Tests:

• Full suite: 223 passed, 28 skipped, 0 failed (coverage gate passed).
• New regression tests (unit in tests/test_comm.py, live-kernel in tests/test_subshells.py, parametrized subshell on/off): fail without the fix, pass with it.