fix: make MessagingClientConf.rateLimit reachable, warn on dead channel knobs

Addresses two review findings on the move PR:

- `rateLimit` was a plain `RateLimitConfig`, but `merge` only copies
  `Opt` fields (`when oField is Opt`), so every override path
  (`LogosDeliveryConf.init`, JSON `messagingOverrides`) silently dropped
  it and the field was always taken from `base` — unsettable by any
  caller. Make it `Opt[RateLimitConfig]` like every other field;
  `MessagingClient.new` falls back to `DefaultRateLimitConfig` (new
  const, rate limiting disabled) when unset. Adds a merge test.

- The channel-level knobs (`rateLimitEnabled` /
  `rateLimitEpochPeriodSec` / `rateLimitMessagesPerEpoch`) are still
  parsed but unread since rate limiting moved to the messaging client,
  so setting them was silently ignored. `ReliableChannelManager.new`
  now logs a deprecation warning when any is set. Full removal remains a
  follow-up API decision.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

logos_delivery/api/conf/messaging_conf.nim

@@ -51,9 +51,10 @@ type MessagingClientConf* = object
     ## Process log format (TEXT or JSON); applied by the kernel on node creation.
   nodeKey* {.name: "nodekey".}: Opt[crypto.PrivateKey]
     ## P2P node private key (64-char hex): stable identity / peerId across restarts.
-  rateLimit*: RateLimitConfig = RateLimitConfig(
-    epochPeriodSec: DefaultEpochPeriodSec, messagesPerEpoch: DefaultMessagesPerEpoch
-  ) ## RLN-epoch transmission budget enforced by the send service.
+  rateLimit*: Opt[RateLimitConfig]
+    ## Per-epoch message rate limit enforced by the send service. `Opt` like
+    ## every other field so `merge` propagates a caller's override; unset falls
+    ## back to `DefaultRateLimitConfig` (rate limiting disabled).
 
 proc applyMode*(conf: var WakuNodeConf, mode: LogosDeliveryMode): ConfResult[void] =
   ## Sets the protocol flags implied by the mode.

logos_delivery/channels/reliable_channel_manager.nim

@@ -32,6 +32,11 @@ proc new*(
     conf: ReliableChannelManagerConf,
     brokerCtx: BrokerContext = globalBrokerContext(),
 ): Result[T, string] =
+  if conf.rateLimitEnabled.isSome() or conf.rateLimitEpochPeriodSec.isSome() or
+      conf.rateLimitMessagesPerEpoch.isSome():
+    warn "channel-level rate-limit config is deprecated and ignored; " &
+      "rate limiting moved to the messaging client (MessagingClientConf.rateLimit)"
+
   return ok(
     T(
       channels: initTable[ChannelId, ReliableChannel](),

logos_delivery/messaging/messaging_client.nim

@@ -25,8 +25,9 @@ proc new*(
   ## The messaging layer chains onto Waku: it drives the underlying Waku kernel
   ## for transport while exposing its own send/recv API.
   let reliability = conf.reliabilityEnabled.get(DefaultP2pReliability)
-  let sendService =
-    ?SendService.new(reliability, waku, RateLimitManager.new(conf.rateLimit))
+  let sendService = ?SendService.new(
+    reliability, waku, RateLimitManager.new(conf.rateLimit.get(DefaultRateLimitConfig))
+  )
   let recvService = RecvService.new(waku)
   return ok(
     T(

logos_delivery/messaging/rate_limit_manager/rate_limit_manager.nim

@@ -32,6 +32,10 @@ const
   DefaultEpochPeriodSec* = 600
   DefaultMessagesPerEpoch* = 1
 
+  DefaultRateLimitConfig* = RateLimitConfig(
+    epochPeriodSec: DefaultEpochPeriodSec, messagesPerEpoch: DefaultMessagesPerEpoch
+  ) ## Used when no rate-limit config is supplied; `enabled` defaults false.
+
 proc new*(T: type RateLimitManager, config: RateLimitConfig): T =
   return
     T(config: config, queue: @[], currentEpochStart: getTime(), sentInCurrentEpoch: 0)

tests/api/test_conf.nim

@@ -112,6 +112,20 @@ suite "MessagingClientConf - merge (override wins)":
       mc.clusterId == Opt.some(2'u16) # override wins
       mc.maxMessageSize == Opt.some("1MB") # base preserved
 
+  test "a rateLimit override propagates through merge":
+    ## Regression: rateLimit must be `Opt` so `merge` copies it; as a plain
+    ## object it was always taken from base, leaving the field unreachable.
+    let overrides = MessagingClientConf(
+      rateLimit: Opt.some(
+        RateLimitConfig(enabled: true, epochPeriodSec: 30, messagesPerEpoch: 5)
+      )
+    )
+    let mc = merge(MessagingClientConf(), overrides)
+    check:
+      mc.rateLimit == overrides.rateLimit
+      mc.rateLimit.get().enabled
+      mc.rateLimit.get().messagesPerEpoch == 5
+
 suite "parseLogosDeliveryConf - JSON parsing":
   test "empty object resolves to a full Core node conf":
     let lc = parseLogosDeliveryConf("{}").valueOr: