Fix Union JSON serialization (#1936)

This doesn't allow JSON deserialization (for that, some rust/runtime
work has to happen) but it does fix serialization to at least work one
way.
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----

> [!IMPORTANT]
> Add JSON serialization support for union types in Go, with
`MarshalJSON` methods and a test file, but deserialization is not
implemented.
> 
>   - **Serialization**:
> - Add `MarshalJSON` method to union types in `types-unions.go.j2` and
`unions.go` for JSON serialization.
> - `UnmarshalJSON` method added but returns "not implemented" error.
>   - **Testing**:
>     - Add `json_test.go` to test JSON serialization round-trip.
> - Includes a commented-out test case for `Union__float__bool`
indicating incomplete functionality.
> 
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Author: tberman

engine/language_client_codegen/src/go/templates/types-unions.go.j2

@@ -1,9 +1,10 @@
 package types
 
 import (
+	"encoding/json"
 	"fmt"
 
-    flatbuffers "github.com/google/flatbuffers/go"
+	flatbuffers "github.com/google/flatbuffers/go"
 	baml "github.com/boundaryml/baml/engine/language_client_go/pkg"
 	"github.com/boundaryml/baml/engine/language_client_go/pkg/cffi"
 )
@@ -47,6 +48,21 @@ func (u {{ union.name }}) BamlTypeName() string {
     return "{{ union.name }}"
 }
 
+func (u {{ union.name }}) MarshalJSON() ([]byte, error) {
+    switch u.variant {
+    {% for variant in union.variants %}
+    case "{{ variant.0 }}":
+        return json.Marshal(u.variant_{{ variant.0 }})
+    {% endfor %}
+    }
+
+    return nil, fmt.Errorf("invalid union variant: %s", u.variant)
+}
+
+func (u *{{ union.name }}) UnmarshalJSON(data []byte) error {
+    return fmt.Errorf("not implemented")
+}
+
 {% for variant in union.variants %}
 func (u *{{ union.name }}) Set{{ variant.0|exported_name }}(v {{ variant.1.name }}) {
     u.variant = "{{ variant.0 }}"