* nd-mcp: re-raise asyncio.CancelledError in reconnect-delay handler

Sonar python:S7497: connect_with_backoff() caught CancelledError during
the reconnect-delay wait but swallowed it with `pass`. A parent-driven
cancellation (e.g., process shutdown) could be lost and the outer
`while True` reconnect loop would continue.

Replace `pass` with `raise` to propagate the cancellation. The
preceding lines already perform local cleanup (cancelling helper
tasks, clearing retry_event), so no additional cleanup is needed
before re-raising.

The similar handler at line 312 is a different pattern (cancel-then-
await on a child task this code just cancelled); it intentionally
suppresses the expected CancelledError from the awaited task and is
addressed in a separate finding.

* nd-mcp: drain cancelled tasks via gather() so outer cancellation propagates

Sonar python:S7497: the cleanup loop after asyncio.wait() cancelled
each pending task and awaited it inside `try/except CancelledError:
pass`. The handler swallowed the expected CancelledError from the
just-cancelled child, but it would also swallow a CancelledError from
the OUTER `connect_with_backoff` coroutine if the parent arrived
mid-await -- breaking shutdown propagation.

Cancel all pending tasks first, then drain them via
`asyncio.gather(..., return_exceptions=True)`. Per-child CancelledError
is collected as a result and ignored; non-cancellation exceptions are
re-raised. If `connect_with_backoff` itself is cancelled while awaiting
the gather, gather propagates the cancellation unconditionally.

Side-benefit: the original sequential loop would skip cancelling later
tasks if an earlier task raised non-CancelledError. The gather form
cancels all up front, so cleanup is complete regardless of outcome.

* nd-mcp: address PR-review findings

Cancel both helper tasks and clear the retry event before re-raising asyncio.CancelledError so reconnect-delay cancellation does not leave dangling state behind.

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Co-authored-by: Costa Tsaousis <costa@netdata.cloud>
(cherry picked from commit 735549f)