[Rust] Ensure unions are referenced with the correct path (#6422)

* Add codegen test for namespaced unions

* [Rust] Handle cross-namespace union use

* [Rust] Test namespace handling

* [Rust] Drop trailing whitespace in codegen

* [Rust] Set flags in generate_code.bat to match .sh

* [C#] Add additional namespace test file

tests/namespace_test/NamespaceA/NamespaceB/UnionInNestedNS.go

@@ -0,0 +1,36 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package NamespaceB
+
+import (
+	"strconv"
+
+	flatbuffers "github.com/google/flatbuffers/go"
+
+	NamespaceA__NamespaceB "NamespaceA/NamespaceB"
+)
+
+type UnionInNestedNST struct {
+	Type UnionInNestedNS
+	Value interface{}
+}
+
+func (t *UnionInNestedNST) Pack(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	if t == nil {
+		return 0
+	}
+	switch t.Type {
+	case UnionInNestedNSTableInNestedNS:
+		return t.Value.(*NamespaceA__NamespaceB.TableInNestedNST).Pack(builder)
+	}
+	return 0
+}
+
+func (rcv UnionInNestedNS) UnPack(table flatbuffers.Table) *UnionInNestedNST {
+	switch rcv {
+	case UnionInNestedNSTableInNestedNS:
+		x := TableInNestedNS{_tab: table}
+		return &NamespaceA__NamespaceB.UnionInNestedNST{ Type: UnionInNestedNSTableInNestedNS, Value: x.UnPack() }
+	}
+	return nil
+}