feat: add protoJsonFormat option to support standard Protobuf JSON message (#1241)

The [Protobuf JSON Mapping
specification](https://protobuf.dev/programming-guides/json/#:~:text=Format%20Description-,Representation%20of%20each%20type,-The%20following%20table)
requires that messages parsers accept both the lowerCamelCase name (or
explicit json_name) and the original proto field name (often snake_case)
when parsing JSON. Before this change, ts-proto's fromJSON generated
method only supported one format based on the snakeToCamel
configuration, typically prioritizing camelCase.

**Mechanism**: This PR introduces a new boolean option: protoJsonFormat.
When enabled:

- snakeToCamel includes json, enforcing typical camelCase keys for
output (toJSON).
- fromJSON is enhanced to check for both the standard JSON name
(camelCase or json_name) and the original proto field name.
- If the two names are identical, no extra check is generated to avoid
code bloat.

Example:
``` protobuf
message Message {
  string my_field = 1;
  string second_field = 2 [json_name = "myCustomName"];
}
```
Generated fromJSON:

```typescript
  fromJSON(object: any): Message {
    return {
      myField: isSet(object.myField)
        ? globalThis.String(object.myField)
        : isSet(object.my_field)
        ? globalThis.String(object.my_field)
        : "",
      secondField: isSet(object.myCustomName)
        ? globalThis.String(object.myCustomName)
        : isSet(object.second_field)
        ? globalThis.String(object.second_field)
        : "",
    };
  }
```
Changes:

- Added protoJsonFormat to Options.
- Updated src/main.ts fromJSON generation generation logic to include
fallback che
- Added comprehensive integration tests in
integration/proto-json-format/.

Fixes #1012

This change is required to adopt the ts-proto library in the [A2A-js
project](https://github.com/a2aproject/a2a-js)

Author: guglielmo-san

README.markdown

@@ -414,6 +414,9 @@ Generated code will be placed in the Gradle build directory.
 
   The default behavior is `keys_json`, i.e. both will be camel cased, and `json_name` will be used if set.
 
+- With `--ts_proto_opt=protoJsonFormat=true` (default), the `toJSON` and `fromJSON`  methods follow the [protoJSON format](https://protobuf.dev/programming-guides/json). This means `toJSON` will output standard lowerCamelCase keys (or custom `json_name` if set). `fromJSON` will accept both the lowerCamelCase key (or `json_name`) AND the original proto field name.
+With `--ts_proto_opt=protoJsonFormat=false`, strict proto3 compliance is disabled. The JSON keys will simply follow the `snakeToCamel` option, and `fromJSON` will only accept that specific format.
+
 - With `--ts_proto_opt=outputEncodeMethods=false`, the `Message.encode` and `Message.decode` methods for working with protobuf-encoded/binary data will not be output.
 
   This is useful if you want "only types".