fix(upb): escape field key names in jsonenc_fieldval when encoding (#31010)

### Summary

`jsonenc_fieldval` in `upb/json/encode.c` writes an object key with `jsonenc_printf(e, "\"%s\":", name)`, where `name` comes from `upb_FieldDef_JsonName` (or `upb_FieldDef_Name` with `upb_JsonEncode_UseProtoNames`). Field names are checked as identifiers when a descriptor is added to a def pool, but `json_name` is copied from the `FieldDescriptorProto` unchanged. A `json_name` that contains a quote, a backslash or a control character therefore reaches the output unescaped. With the tests added here, a field whose `json_name` is

```
a":1,"b
```

is encoded as `{"a":1,"b":1}`, which is a two-key object, and a `json_name` containing a newline, a backslash or a tab produces invalid JSON. Values, map keys, struct keys and enum names in the same encoder already go through `jsonenc_string` or `jsonenc_stringbody`.

### Fix

In `jsonenc_fieldval`, emit the key with `jsonenc_string`, followed by `:`. The extension branch, which formats the full name in brackets, now passes the name through `jsonenc_stringbody`. Extension full names are identifier-checked by the def builder, so the extension change has no observable effect for descriptors the def pool accepts; it only makes the two branches consistent. It can be dropped if you prefer the smaller change.

This change does not touch `jsonenc_fieldpath`.

### Relationship to #28678

#28678 makes the same `json_name` change in `jsonenc_fieldval`. It also adds a character allow-list to `jsonenc_fieldpath`. Applied to current main, that allow-list makes the existing `EncodeFieldMaskEscapedPaths` fail, because that test expects paths containing a newline, a tab and a quote to encode and be escaped. This change leaves `jsonenc_fieldpath` alone and `EncodeFieldMaskEscapedPaths` keeps passing. If #28678 lands first, the `jsonenc_fieldval` part of this change is redundant.

### Testing

Added to `upb/json/encode_test.cc`, with a new `JsonNameEscape` message in `upb/json/test.proto`:

- `EncodeJsonNamePositiveControl` (also with `upb_JsonEncode_UseProtoNames`)
- `EncodeJsonNameWithQuotesAndBackslashesIsEscaped` (also decodes the output with `upb_JsonDecode` and checks the field round-trips)
- `EncodeJsonNameWithControlCharactersIsEscaped`

```
bazel test //upb/json:encode_test
```

passes with the change (22 tests, including `EncodeFieldMaskEscapedPaths`). Without the change, the two escaping tests fail and the others pass. Built with clang 22.1.8 and Bazel 8.7.0 on x86_64 Linux, also under ASan and UBSan.

AI assistance was used in drafting and validating this change; the patch and tests were reviewed and verified by the author.

Closes #31010

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/31010 from professor-moody:fix-upb-json-encode-field-key-escaping ff14f061c3b397112c241067e99e1e80aa265c28
PiperOrigin-RevId: 1015144727