When `management_agent.disable_metrics_collector` and
`management.disable_stats` are both set to `true`, the HTTP API returns
minimal queue information that excludes policy-related fields and
`delivery_limit`. This causes `policy`, `operator_policy`,
`effective_policy_definition`, and `delivery_limit` to be `null` in API
responses, even though these are configuration metadata rather than
statistics.

This change adds these four fields to the `format/2` function in both
`rabbit_classic_queue` and `rabbit_quorum_queue` modules. The fields now
appear in the type-specific formatting that runs regardless of metrics
collection status. The three policy fields use existing `i/2` function
implementations that call `rabbit_policy` module functions. For
`delivery_limit`, this change adds an `i(delivery_limit, Q)` function to
`rabbit_quorum_queue` that extracts the delivery limit from the queue's
`x-delivery-limit` argument, returning `unlimited` when not set.

Fixes #15182

…re enabled

`rabbit_queue_type:format/2` now conditionally emits policy fields only
when management stats are disabled: the stats augmentation
code path provides them.

1. `format/2` now conditionally emits policy fields
   based on whether the management plugin stats are enabled
2. `combine/2` acts as the next layer of protection
   against duplicate keys: it drops the `format/2`-originating
   keys that are duplicates
3. `prepare_for_encoding/1` is the new name for `format_nulls/1`
   that reflects its expanded scope: another deduplication pass

References #15182.