fix(rabbitmq): bind handler queue to every handled-message exchange under FromHandlerType

[jeremydmiller]

Closes #2681.

Summary

• Under UseConventionalRouting(NamingSource.FromHandlerType), when a single handler type accepts two or more message types (typical for sagas or any aggregate-style handler), the handler-named queue ended up bound to only the FIRST message type's exchange. Messages published to the remaining exchanges silently never reached the handler queue.
• Root cause: RabbitMqMessageRoutingConvention.ApplyListenerRoutingDefaults short-circuited on queue.HasBindings. The base MessageRoutingConvention.DiscoverListeners calls this method once per (handlerType, messageType) pair under FromHandlerType naming — so the second pass saw the binding the first pass had just added and silently skipped its own.
• The original guard's intent was protecting user-configured custom bindings (set up in the ConfigureListeners lambda, which runs before ApplyListenerRoutingDefaults) from being double-bound on top of by the convention. Narrowing the guard to per-exchange dedup preserves that protected case while letting the multi-message-type case bind correctly.

What changed

src/Transports/RabbitMQ/Wolverine.RabbitMQ/RabbitMqMessageRoutingConvention.cs:23-50:

// before
if (!queue.HasBindings)
{
    var identifier = _identifierForSender(messageType);
    if (identifier is null) return;
    var name = transport.MaybeCorrectName(identifier);
    var exchange = transport.Exchanges[name];
    queue.BindExchange(exchange.Name, exchange.Name);
}

// after
var identifier = _identifierForSender(messageType);
if (identifier is null) return;
var name = transport.MaybeCorrectName(identifier);
var exchange = transport.Exchanges[name];

if (queue.Bindings().Any(b => b.ExchangeName == exchange.Name))
{
    return; // already bound to THIS exchange — don't stack
}

queue.BindExchange(exchange.Name, exchange.Name);

The new guard is strictly weaker than the old one — every case the old guard silenced still gets silenced. The bug case is the only behavioural delta.

Tests

• Bug_2681_handler_type_naming_binds_all_exchanges.handler_queue_is_bound_to_every_handled_message_exchange — primary reproducer. A single MultiMessageHandler handles MultiMessageFoo + MultiMessageBar; pre-fix the handler queue was bound only to whichever message-type exchange got processed second. Verified red on main (queue bound only to MultiMessageBar); green with the fix.
• Bug_2681_handler_type_naming_binds_all_exchanges.both_message_exchanges_were_created — sanity check that both exchanges actually get registered (the bug was purely on the queue-binding side).
• Bug_2681_custom_bindings_are_not_double_added.user_custom_binding_is_not_double_added_by_the_convention — guard that the new per-exchange dedup still suppresses the default convention binding when a user has pre-configured a custom binding to the same exchange via ConfigureListeners + BindToExchange<TMessage>(...).

Test plan

• Reproducer red on main, green with fix
• Full ConventionalRouting suite — 31/31 passed on net9.0

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