[default values] Spark catalog itest for ReadBridge initial-default overlay

[cbb330]

Summary

Stacked on #679. Spark catalog itest that runs create → insert → ADD COLUMN through the OpenHouse catalog and asserts ReadBridge stamped initial-default on Spark's Iceberg table.

OSS has no deployment ColumnDefaultsSource; the local-server fixture supplies one for d1_column_default so the client overlay is exercised. SQL fill/filter cases are present but @Disabled until OpenHouse bumps Iceberg past 1.2.0.20 (linkedin/iceberg#267–269): that runtime's Spark parquet/ORC readers do not fill initial-default.

Changes

• Client-facing API Changes
• Internal API Changes
• Bug Fixes
• New Features
• Performance Improvements
• Code Style
• Refactoring
• Documentation
• Tests

Adds ColumnDefaultSparkITest and a local-server ColumnDefaultsSource stub (SparkItestColumnDefaults) imported by SpringH2TestApplication.

Testing Done

• Manually Tested on local docker setup. Please include commands ran, and their output.
• Added new tests for the changes made.
• Updated existing tests to reflect the changes made.
• No tests added or updated. Please explain why. If unsure, please feel free to ask for help.
• Some other form of testing like staging or soak time in production. Please explain.

./gradlew :integrations:spark:spark-3.1:openhouse-spark-itest:catalogTest --tests com.linkedin.openhouse.spark.catalogtest.ColumnDefaultSparkITest (JDK 11). Overlay test passed; SQL fill test remains disabled.

Additional Information

• Breaking Changes
• Deprecations
• Large PR broken into smaller PRs, and PR plan linked in the description.

Stack

1. #645 — substrate (seam + encode + decode hook)
2. #668 — decode off Iceberg’s retry path
3. #674 — policy / ramp in OpenHouse
4. #678 — sanitize-on-write
5. #679 — ReadBridge.apply
6. This PR — Spark catalog itest (overlay)

Depends on #679. Enable columnDefaultBackfillViaReadBridge after the Iceberg bump that includes reader fill.