fix(replication): accept remote target healthCheckDuration nanoseconds (#5754)

* test(replication): accept madmin nanosecond healthCheckDuration payloads

Red-phase TDD tests for P0-7: mc 'replicate add' sends the madmin default
healthCheckDuration=60s as a Go time.Duration nanosecond integer
(60000000000), which RustFS currently rejects as an unsupported field and
would misread as seconds. Also pins the defensive seconds-or-nanos read
for persisted bucket-targets metadata and the capability contract listing
healthCheckDuration as writable.

Currently failing (red):
- remote_target_request_accepts_go_duration_wire_values
- remote_target_request_accepts_legacy_seconds_health_check
- remote_target_health_check_duration_is_declared_writable
- bucket_target_reads_go_nanosecond_durations_defensively
- runtime_capabilities_response_reports_missing_topology_before_storage_init

* fix(replication): accept remote target healthCheckDuration nanoseconds

mc 'replicate add' always sends the madmin default healthcheck-seconds=60
serialized as a Go time.Duration nanosecond integer (60000000000), so the
default mc link-creation path (and 'mc replicate update') failed with
InvalidRequest. Move healthCheckDuration from the unsupported to the
writable remote-target field list; the capability contract in the runtime
capabilities response follows the constants automatically.

Fix the unit mismatch in both directions:
- Request parsing and persisted bucket-targets reads decode the value
  defensively: below 10^7 it is legacy RustFS seconds, otherwise Go
  time.Duration nanoseconds (also covers MinIO-written metadata).
  totalDowntime shares the same wire shape and gets the same handling.
- The list-remote-targets admin response re-encodes only these two fields
  as nanoseconds via a dedicated serialization path, leaving the persisted
  seconds-based wire format untouched for existing readers.

The per-target health-check interval is accepted for mc compatibility but
not yet applied; the heartbeat keeps its global env-configured interval,
and the explicit 'healthcheck' update op stays rejected. disableProxy,
edge, and edgeSyncBeforeExpiry remain explicitly rejected.

crates/replication/src/config.rs

@@ -73,9 +73,13 @@ pub const REMOTE_TARGET_WRITABLE_FIELDS: &[&str] = &[
     "storage_class",
     "skipTlsVerify",
     "caCertPem",
+    // Accepted for mc compatibility (mc `replicate add` always sends the
+    // madmin default of 60s); the per-target health-check interval is not
+    // yet applied — the heartbeat keeps its global env-configured interval.
+    "healthCheckDuration",
 ];
 
-pub const REMOTE_TARGET_UNSUPPORTED_FIELDS: &[&str] = &["disableProxy", "healthCheckDuration", "edge", "edgeSyncBeforeExpiry"];
+pub const REMOTE_TARGET_UNSUPPORTED_FIELDS: &[&str] = &["disableProxy", "edge", "edgeSyncBeforeExpiry"];
 
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct ObjectOpts {