rabbit_stream_coordinator: Make new_stream command idempotent (backport #15706)

[mergify]

This is similar to #14884 but for new_stream.

Although its unlikely from the calling code, the stream coordinator can end up with two {new_stream, StreamId, #{}} commands in its log for the same StreamId. When handling the second new_stream it will then warning-log that the stream can't be updated, like so:

2026-03-10 17:31:38.897507+00:00 [warning] <0.14468.0> rabbit_stream_coordinator failed to update stream:
2026-03-10 17:31:38.897507+00:00 [warning] <0.14468.0> function_clause
2026-03-10 17:31:38.897507+00:00 [warning] <0.14468.0> [{rabbit_stream_coordinator,update_stream0,[#{index => 51,system_time => 1773163898892,reply_mode => await_consensus,machine_version => 6,...},{new_stream,[95|...],#{}},{stream,[...],...}],[{file,[114|...]},{line,1397}]},{rabbit_stream_coordinator,update_stream,3,[{file,[...]},{line,...}]},{rabbit_stream_coordinator,apply,3,[{file,...},{...}]},{ra_server,apply_with,2,[{...}|...]},{ra_log,fold,5,[...]},{ra_server,apply_to,5,...},{ra_server,evaluate_commit_index_follower,...},{ra_server,...}]

update_stream0/3 only has a function clause for when there is no stream (i.e. undefined,

rabbitmq-server/deps/rabbit/src/rabbit_stream_coordinator.erl

Lines 1397 to 1420 in 0b951b8

	update_stream0(#{system_time := _} = Meta,
	{new_stream, StreamId, #{leader_node := LeaderNode,
	queue := Q}}, undefined) ->
	#{nodes := Nodes} = Conf = amqqueue:get_type_state(Q),
	%% this jumps straight to the state where all members
	%% have been stopped and a new writer has been chosen
	E = 1,
	QueueRef = amqqueue:get_name(Q),
	Members = maps:from_list(
	[{N, #member{role = case LeaderNode of
	N -> {writer, E};
	_ -> {replica, E}
	end,
	state = {ready, E},
	%% no members are running actions
	current = undefined}
	} || N <- Nodes]),
	#stream{id = StreamId,
	epoch = E,
	nodes = Nodes,
	queue_ref = QueueRef,
	conf = Conf,
	members = Members,
	reply_to = maps:get(from, Meta, undefined)};

). I think it's reasonable to have the new_stream command become idempotent and effectively skip it if it is in the log twice.

I haven't been able to reproduce the scenario where this happened. Next time I will dump Khepri and the coordinator's logs to see what sequence of commands lead to this.

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This is an automatic backport of pull request #15706 done by Mergify.