fix(admin): expose per-target disableProxy through remote target admin API (#6376)

The read-proxy selector already honors a target's disable_proxy flag
(PR #6172), but the admin API still rejected the field, so the only way
to set it was importing a MinIO-written bucket-targets.json.

- move disableProxy from REMOTE_TARGET_UNSUPPORTED_FIELDS to
  REMOTE_TARGET_WRITABLE_FIELDS (set-remote-target create accepts it)
- add TargetUpdateOp::Proxy so set-remote-target?update=true&proxy=true
  overlays only the proxy group (MinIO TargetUpdateType parity)
- bump REMOTE_TARGET_CAPABILITY_CONTRACT_VERSION 1 -> 2 and update the
  runtime capability pin tests
- keep edge/edgeSyncBeforeExpiry rejected (no implementation behind them)
- pin that a published TargetClient carries disable_proxy, the field the
  proxy-target selector consults

Refs rustfs/backlog#1950

crates/replication/src/config.rs

@@ -60,7 +60,9 @@ pub const REPLICATION_READ_ONLY_HISTORICAL_FIELDS: &[&str] = &[
     "Destination.ReplicationTime",
 ];
 
-pub const REMOTE_TARGET_CAPABILITY_CONTRACT_VERSION: u32 = 1;
+// v2: disableProxy moved from unsupported to writable (per-target read-proxy
+// opt-out is accepted by set-remote-target and the `proxy` update op).
+pub const REMOTE_TARGET_CAPABILITY_CONTRACT_VERSION: u32 = 2;
 
 pub const REMOTE_TARGET_WRITABLE_FIELDS: &[&str] = &[
     "sourcebucket",
@@ -83,9 +85,12 @@ pub const REMOTE_TARGET_WRITABLE_FIELDS: &[&str] = &[
     // madmin default of 60s); the per-target health-check interval is not
     // yet applied — the heartbeat keeps its global env-configured interval.
     "healthCheckDuration",
+    // Per-target read-proxy opt-out, consumed by the proxy-target selector
+    // (contract v2; previously only importable via MinIO bucket-targets.json).
+    "disableProxy",
 ];
 
-pub const REMOTE_TARGET_UNSUPPORTED_FIELDS: &[&str] = &["disableProxy", "edge", "edgeSyncBeforeExpiry"];
+pub const REMOTE_TARGET_UNSUPPORTED_FIELDS: &[&str] = &["edge", "edgeSyncBeforeExpiry"];
 
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct ObjectOpts {