Handle receiver join failures gracefully in ping expectations

[Copilot]

When receivers fail to join the group chat (timeout/exception), the script incorrectly expected all original receivers to receive ping messages, causing inflated loss percentages and incorrect exit codes.

Changes

• wait_for_receivers_to_join(): Returns (joined_indices, join_errors) tuple instead of just count

  • Tracks which receivers failed with descriptive error messages including receiver addresses

• perform_ping(): Filters receiver list to only successfully joined receivers

  • N/M ratio display uses actual joined count
  • Adds --- join errors --- section at end of output

• main(): Exit code now based on len(pinger.receivers) (joined) instead of args.numrecipients (requested)

Example output when 1 of 3 receivers fails to join

# WARNING: Only 2/3 receivers joined the group
CMPING relay.example.org(...) -> relay.example.org(group with 2 receivers) count=10
64 bytes ME -> relay.example.org -> relay.example.org -> ME seq=0 time=245.76ms 2/2
...
--- timing statistics ---
account setup: 1.23s
group join: 30.12s
message send/recv: 11.45s
--- join errors ---
✗ receiver user3@relay.example.org did not join group within 30s

Original prompt

When receivers failed to join, then only expect so many receivers in the ping group chat to receive the ping message, not the original number. Adjust the script to only account for receivers that successfully joined the group chat, and note down the error of receivers not joining at the end of the output.

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