feat(frontend): allow users to confirm if background_ddl is enabled for a sink

[Standing-Man]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

• Closes Part of Tracking: Improve management and observability of creating stream jobs #19084.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note

[Standing-Man]

@kwannoel PTAL.

[Copilot]

Pull request overview

This PR adds a background_ddl column to the rw_catalog.rw_sinks system catalog table to allow users to confirm whether background_ddl was enabled when a sink was created. This mirrors the existing background_ddl field already present in the rw_materialized_views system catalog.

Changes:

• Added background_ddl boolean field to the RwSink struct in the system catalog
• Populated the field by comparing sink.create_type with CreateType::Background
• Added e2e test to verify the feature works correctly for both foreground and background sink creation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/frontend/src/catalog/system_catalog/rw_catalog/rw_sinks.rs	Added background_ddl field to RwSink struct and populated it based on create_type
e2e_test/sink/check_sink_background.slt	Added e2e test validating background_ddl field for sinks created with/without background_ddl setting
ci/scripts/e2e-sink-test.sh	Integrated the new test into the CI test suite

[Standing-Man]

Hi @kwannoel, please take a look when you have time.