fix(ec): bring ec.encode worker and EC/volume helpers to parity with shell (#9599)

* refactor(volume): extract replica sync/select into shared volume_replica package

Move the volume replica reconciliation helpers (status, union builder,
SyncAndSelectBestReplica, ReadNeedleMeta) out of the shell into a new
weed/storage/volume_replica package so both the shell (ec.encode, volume.tier.move,
volume.check.disk) and the EC encode worker can reuse them. No behavior change.

* fix(ec): bring ec.encode worker to parity with the shell

- Sync replicas and encode the most-complete one (via the shared
  volume_replica.SyncAndSelectBestReplica) instead of a possibly-stale replica,
  marking all replicas readonly first. Prevents silent data loss when a stale
  replica is encoded and the originals deleted.
- Skip remote/tiered volumes in detection (shell ec.encode excludes them).
- Min-node safety gate: refuse to encode when cluster nodes < parity shards.
- Align default thresholds with the shell (fullness 0.95, quiet 1h).

* fix(vacuum): plugin path honors min_volume_age_seconds override

deriveVacuumConfig hard-coded MinVolumeAgeSeconds=0, dropping any configured
value. Read it from worker config (default 0, matching the shell/master vacuum
which has no age gate) so an explicit override is honored.

* address review feedback

- config.go: align GetConfigSpec schema defaults (quiet_for_seconds=3600,
  fullness_ratio=0.95) with the runtime defaults so UI/bootstrap flows match the
  shell (coderabbitai).
- ec_task.go: roll back readonly when markReplicasReadonly fails partway, so
  already-marked replicas don't stay readonly (coderabbitai).
- volume_replica: pass the caller's replica statuses into buildUnionReplica instead
  of re-fetching them, and skip the per-needle ReadNeedleMeta RPC when the source
  replica is read-only (gemini-code-assist).

* test(plugin_workers/ec): make fixtures eligible under the new defaults

The default EC encode thresholds were raised to match the shell (fullness 0.95,
quiet 1h), but the plugin-worker integration fixtures still used 90%-full /
10-minute-old volumes, so detection found no eligible volumes and the tests failed
in CI. Bump the eligible fixtures to 96% full and 2h old.

test/plugin_workers/erasure_coding/detection_test.go

@@ -207,8 +207,10 @@ func buildVolumeListResponse(t *testing.T, spec topologySpec, volumeID uint32) *
 	t.Helper()
 
 	volumeSizeLimitMB := uint64(100)
-	volumeSize := uint64(90) * 1024 * 1024
-	volumeModifiedAt := time.Now().Add(-10 * time.Minute).Unix()
+	// Exceed the default fullness (0.95) and quiet (1h) thresholds so volumes are
+	// EC-eligible.
+	volumeSize := uint64(96) * 1024 * 1024
+	volumeModifiedAt := time.Now().Add(-2 * time.Hour).Unix()
 
 	diskTypes := spec.diskTypes
 	if len(diskTypes) == 0 {

test/plugin_workers/erasure_coding/large_topology_test.go

@@ -29,9 +29,11 @@ func TestErasureCodingDetectionLargeTopology(t *testing.T) {
 	}
 
 	nodesPerRack := serverCount / rackCount
-	eligibleSize := uint64(90) * 1024 * 1024
+	// Eligible volumes must exceed the default fullness (0.95) and quiet (1h)
+	// thresholds; ineligible ones fall below the fullness threshold.
+	eligibleSize := uint64(96) * 1024 * 1024
 	ineligibleSize := uint64(10) * 1024 * 1024
-	modifiedAt := time.Now().Add(-10 * time.Minute).Unix()
+	modifiedAt := time.Now().Add(-2 * time.Hour).Unix()
 
 	volumeID := uint32(1)
 	dataCenters := make([]*master_pb.DataCenterInfo, 0, 1)