Fix routing for empty binding key to topic exchange (backport #16271)

[mergify]

Fixes #16221

What?

Fix a bug in the Khepri v4 topic trie projection where messages published with an empty routing key to a topic exchange were incorrectly routed to queues bound to other topic exchanges with an empty binding key.

Why?

The MQTT 5.0 spec states:

All Topic Names and Topic Filters MUST be at least one character long

In contrast, the AMQP 0.9.1 spec state:

The routing key used for a topic exchange MUST consist of zero or more words
delimited by dots. Each word may contain the letters A-Z and a-z and digits 0-9.
The routing pattern follows the same rules as the routing key with the addition
that * matches a single word, and # matches zero or more words

Hence zero words, i.e. empty routing keys and empty binding keys, are expliclity allowed for the topic exchange type.

In the Khepri v4 projection, the topic trie used a global root atom as the initial node ID for all topic exchanges. When a binding was created with an empty binding key (<<>>), split_topic_key_binary/1 returned an empty list ([]). The trie traversal stopped immediately at the root, meaning the binding was inserted into the rabbit_khepri_topic_binding_v4 ETS table with the global root atom as the LeafNodeId.

Because the LeafNodeId lacked any exchange-specific context, all empty bindings across all topic exchanges were attached to the exact same global root node. Consequently, when a message was published with an empty routing key to any topic exchange, the routing logic (trie_bindings/3) would scan the ETS table starting at the global root node and incorrectly match bindings belonging to completely unrelated topic exchanges.

How?

To fix this and ensure exchange isolation, the conceptual root of the trie was changed from the global root atom to an exchange-specific tuple: {root, XSrc} (where XSrc is {VHost, ExchangeName}).

• rabbit_khepri:trie_follow_down_create/3 and trie_follow_down_get_path/3 now initialize their trie traversal with {root, XSrc}.
• rabbit_db_topic_exchange:trie_match/5 now initiates routing from {root, {VHost, XName}}.

This structural change guarantees that empty bindings are isolated per exchange in the bindings ETS table, completely resolving the cross-exchange leakage while fully supporting AMQP 0-9-1's allowance for empty binding keys without breaking backward compatibility.

TODOs

• New Khepri projection version (see 2nd commit)
• Release notes

  ---

  This is an automatic backport of pull request Fix routing for empty binding key to topic exchange #16271 done by Mergify.