[Bug][Connector-JSON] Schema defaultValue is ignored during JSON deserialization (fields remain null)

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/seatunnel/issues?q=is%3Aissue+label%3A%22bug%22) and found related but **different-root-cause** reports:
- [#8759](https://github.com/apache/seatunnel/issues/8759) — Oracle CDC → Kafka: Debezium struct default value used when column is null (Debezium deserialization path).
- [PR #7950](https://github.com/apache/seatunnel/commit/3b432125a) — fixed the Debezium path via `struct.getWithoutDefault(...)`; the **JSON format** deserializer (`seatunnel-format-json`) is a separate code path and was NOT touched.
- [PR #6247](https://github.com/apache/seatunnel/pull/6247) — fixed `Object.class` option value parsing so `defaultValue` config can be read correctly.
- No existing issue reports the JSON format deserializer ignoring `schema.columns[].defaultValue`.

### What happened

When a source connector using the **JSON format** (Kafka, Pulsar, HTTP, MongoDB, Elasticsearch, …) configures `schema { columns = [ { name = "GROUPID", type = "int", nullable = false, defaultValue = 0 } ] }`, the `defaultValue` is **parsed and stored on the `Column`** but **never applied** during deserialization: if the JSON message is missing the field or contains an explicit `null`, the row field stays `null` instead of the configured default value.

Expected: `defaultValue = 0` is used when the JSON field is missing or `null`.
Actual: the field is `null`; the `defaultValue` config is silently ignored.

### SeaTunnel Version

2.3.13 (verified locally). **Also reproducible on current upstream `dev`** (checked 2026-08-03, `6bf786bee7`).

### SeaTunnel Config

```hocon
source {
Kafka {
bootstrap.servers = "localhost:9092"
topic = "test"
format = json
schema {
columns = [
{ name = "GROUPID", type = "int", nullable = false, defaultValue = 0 }
{ name = "name", type = "string", nullable = false, defaultValue = "unknown" }
]
}
}
}

sink {
Console {}
}
```

### Running Command

```shell
./bin/seatunnel.sh --config ./config/xxx.conf
```

### Error Exception

No error. The job runs, but fields configured with `defaultValue` come out as `null` when the JSON message omits them or sets them to `null` — a silent data-integrity bug.

### Root cause (confirmed by code inspection on upstream dev)

The `defaultValue` is lost on the way from `CatalogTable` to the row converter:

```
CatalogTable (contains Column metadata incl. defaultValue)
↓ getSeaTunnelRowType()
SeaTunnelRowType (fieldNames[] + fieldTypes[] only) ← defaultValue dropped here
↓
JsonToRowConverters (no access to Column[] / defaultValue)
↓
convertField(): field == null → returns null (defaultValue never consulted)
```

Concretely, on upstream `dev` (same as 2.3.13):

1. `JsonToRowConverters.convertField(...)` (lines ~474-486) — for a missing field it only checks `failOnMissingField` and otherwise returns `null`; it has **no `Column[]` metadata** and never checks `getDefaultValue()`:

```java
private Object convertField(
JsonToObjectConverter fieldConverter, String fieldName, JsonNode field) {
if (field == null) {
if (failOnMissingField) {
throw new IllegalArgumentException(...);
} else {
return null; // defaultValue never applied
}
} else {
return fieldConverter.convert(field, fieldName);
}
}
```

2. `JsonDeserializationSchema` (both constructors, lines ~82 and ~103) builds `new JsonToRowConverters(failOnMissingField, ignoreParseErrors)` and never passes `catalogTable.getTableSchema().getColumns()` to the converters.

3. The configuration side works correctly: `ReadonlyConfigParser` (line ~139) reads `columnConfig.get(ConnectorCommonOptions.DEFAULT_VALUE)` into `PhysicalColumn.of(...)`, and `Column.getDefaultValue()` exists in `seatunnel-api`. The API contract explicitly supports default values — the deserialization side just never uses them.

### Proposed fix (implemented & verified locally on 2.3.13)

Minimal, backward-compatible, root-cause fix:

- `JsonToRowConverters`: add a `Column[] columns` field and a new constructor `JsonToRowConverters(boolean failOnMissingField, boolean ignoreParseErrors, Column[] columns)`; add a `convertField(converter, fieldName, field, fieldIndex)` overload that consults `columns[fieldIndex].getDefaultValue()` when the field is missing or `null`, falling back to the original `failOnMissingField`/`null` behavior when no default value is configured.
- `JsonDeserializationSchema`: extract `Column[]` from `catalogTable.getTableSchema().getColumns()` (with null-safety) and pass it to the new constructor.
- New test `JsonDefaultValueTest`: missing field → defaultValue; explicit `null` → defaultValue; no defaultValue → `null` (backward compatibility); string-typed default values.

Verification on 2.3.13: 3 new unit tests + full `seatunnel-format-json` module (46 tests) all pass; `spotless:apply` clean.

### Zeta or Flink or Spark Version

zeta (format module is engine-agnostic; affects all of them)

### Java or Scala Version

Java 8 / 11 (both OK)

### Screenshots

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR! (fix already implemented and verified locally; will port to `dev`)

### Code of Conduct

Start with JsonToRowConverters and JsonDeserializationSchema in the seatunnel-format-json module, then review the proposed JsonDefaultValueTest cases. Run the module's existing tests first; done means missing and explicit null JSON fields use configured defaults, while fields without defaults retain the existing null behavior.

Written by the indexing model from the issue text.