REST: Allow struct default values in schema fields

[akashmalbari]

Fixes #16596.

This updates REST/OpenAPI schema field default handling so initial-default and write-default can represent valid Iceberg struct defaults such as {}.

The change keeps expression literal values restricted to primitive values while allowing schema field defaults to use the broader default-value representation required by the Iceberg v3 spec.

Added regression coverage for:

• struct field defaults represented as {}
• nested field defaults inside that struct
• round-tripping schema/default JSON
• REST create-table/schema parsing with struct defaults

Validation:

• JAVA_HOME=/opt/homebrew/Cellar/openjdk@17/17.0.19/libexec/openjdk.jdk/Contents/Home ./gradlew :iceberg-core:test --tests org.apache.iceberg.TestSchemaParser --tests org.apache.iceberg.rest.requests.TestCreateTableRequest --stacktrace
• JAVA_HOME=/opt/homebrew/Cellar/openjdk@17/17.0.19/libexec/openjdk.jdk/Contents/Home ./gradlew :iceberg-open-api:check --stacktrace
• JAVA_HOME=/opt/homebrew/Cellar/openjdk@17/17.0.19/libexec/openjdk.jdk/Contents/Home ./gradlew spotlessApply
• JAVA_HOME=/opt/homebrew/Cellar/openjdk@17/17.0.19/libexec/openjdk.jdk/Contents/Home ./gradlew spotlessCheck

[nssalian]

@akashmalbari thanks for contributing the fix. Please get CI to a healthy state. Happy to review soon after.

[akashmalbari]

Thanks! I’m looking into the failing CI now and will push a follow-up shortly.

[akashmalbari]

Thanks! I pushed a follow-up regenerating open-api/rest-catalog-open-api.py.

The OpenAPI spec validates locally with:
cd open-api && make lint && make generate

The Kafka Connect failure appears unrelated to this PR: it failed while resolving dependencies from Maven Central with HTTP 403 responses.

The CI is in a healthy state.