[go] JSON deserialize Union fix (#1954)

Talked this through w/ @hellovai today, and this approach makes sense.

Walks the union variants and just tries to deserialize until it is
successful or fails completely.
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----

> [!IMPORTANT]
> Implements JSON deserialization for union types in Go by attempting to
unmarshal each variant until successful, with tests added for
verification.
> 
>   - **Behavior**:
> - Implements `UnmarshalJSON` for union types in `types-unions.go.j2`
and `unions.go`.
> - Attempts to unmarshal each variant until successful, otherwise
returns an error.
>   - **Tests**:
> - Adds test cases for `Union__float__bool` in `json_test.go` to verify
round-trip serialization and deserialization.
> 
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Author: tberman

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

@@ -60,7 +60,17 @@ func (u {{ union.name }}) MarshalJSON() ([]byte, error) {
 }
 
 func (u *{{ union.name }}) UnmarshalJSON(data []byte) error {
-    return fmt.Errorf("not implemented")
+    var err error
+    {% for variant in union.variants %}
+    err = json.Unmarshal(data, &u.variant_{{ variant.0 }})
+    if err == nil {
+        u.variant = "{{ variant.0 }}"
+        return nil
+    } else {
+        u.variant_{{ variant.0 }} = nil
+    }
+    {% endfor %}
+    return fmt.Errorf("invalid union variant: %s", string(data))
 }
 
 {% for variant in union.variants %}