fix(filer/postgres): default to ON CONFLICT upsert to keep tx alive (#9709)

* fix(filer/postgres): default to ON CONFLICT upsert to keep tx alive

A KvPut from the inode-index secondary write could fail with 23505
(duplicate key) inside a rename's transaction, after which the next
statement returned 25P02 and rename surfaced to FUSE as EIO. Default
the postgres upsert query when enableUpsert=true so INSERTs are
idempotent; the enableUpsert=false escape hatch is preserved for
non-PG-compatible backends.

* fix(filer/mysql): default to ON DUPLICATE KEY UPDATE upsert

Same shape as the postgres default: when enableUpsert=true but no
upsertQuery is configured, install a sensible default so the
inode-index KvPut does not waste a duplicate-key roundtrip on every
entry write. Uses the VALUES() form so the default works on MariaDB
and MySQL >=5.7; the MySQL 8.0.19 row-alias form is left to explicit
config.

* fix(filer): default enableUpsert=true for sql stores

The default-template fallback only kicks in when enableUpsert=true,
so minimal configs that omit the flag entirely were still exposed.
Default it on for postgres/postgres2/mysql/mysql2; an explicit false
in filer.toml still wins because SetDefault only fills absent keys.

weed/filer/postgres/postgres_sql_gen_test.go

@@ -0,0 +1,25 @@
+package postgres
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDefaultUpsertQueryIsConflictSafe(t *testing.T) {
+	gen := &SqlGenPostgres{UpsertQueryTemplate: DefaultUpsertQuery}
+	got := gen.GetSqlInsert("filemeta")
+	if !strings.Contains(got, "ON CONFLICT") {
+		t.Fatalf("expected ON CONFLICT in default upsert, got: %s", got)
+	}
+	if !strings.Contains(got, `"filemeta"`) {
+		t.Fatalf("expected quoted table name, got: %s", got)
+	}
+}
+
+func TestEmptyUpsertTemplateFallsBackToPlainInsert(t *testing.T) {
+	gen := &SqlGenPostgres{}
+	got := gen.GetSqlInsert("filemeta")
+	if strings.Contains(got, "ON CONFLICT") {
+		t.Fatalf("plain INSERT path should not contain ON CONFLICT, got: %s", got)
+	}
+}