test(mqtt): extract + test remaining inline MQTT decision points (Phase 6)

Close the host-testable gaps named in Phase 6 of STABILITY_TESTABILITY_HANDOFF.md
by moving the last inline decision logic into the pure, host-tested policy seams:

- WiFi STA reconnect backoff: extract the inline ladder + wrap-safe timing from
  handleWiFiConnection() into MQTTConnectionPolicy::{wifiReconnectBackoffMs,
  wifiReconnectDue,nextWifiBackoffAttempt}. Behavior-preserving (elapsedMs is the
  wrap-safe form of the old ULONG_MAX branch); ladder/clamp/attempt-cap unchanged.
- Publication outcome pairing: name the (packet, raw) -> delivered contract as
  MQTTPacketQueuePolicy::queuedPacketPublished() and wire both queue-drain sites;
  partial success = completed, not retried.
- Freeze MQTTPublicationType enum values in a test (the bridge-side MQTTMessageType
  alignment is already enforced by a compile-time static_assert).

Adds host tests for all three (exact boundaries + millis() rollover). Native suite
green (13 dirs); non-PSRAM observer firmware smoke build compiles.

WebConfig batch/reboot/stop state-machine extraction and queue-orchestration
coverage remain open (tracked in the Phase 6 status).

src/helpers/MQTTPacketQueuePolicy.h

@@ -87,6 +87,17 @@ struct RetryDecision {
   uint32_t next_retry_ms;
 };
 
+// A queued packet counts as delivered if EITHER its structured-packet publish
+// or its raw-frame publish reached at least one slot. Partial success (one
+// succeeds while the other fails or was not attempted) is still success — the
+// packet completes and is not retried. This is the (packet, raw) outcome pairing
+// fed to retryDecision(); naming it keeps the "partial publish = done" contract
+// explicit and host-tested rather than inline in the bridge's queue drain.
+static inline bool queuedPacketPublished(bool packet_published,
+                                         bool raw_published) {
+  return packet_published || raw_published;
+}
+
 static inline RetryDecision retryDecision(bool any_published,
                                           uint8_t retry_attempts,
                                           uint32_t now) {