support RisingWave jsonb schema queries

[Dharshini-RS03]

Description

Fixes RisingWave PostgreSQL object-browser schema reads by falling back from unsupported json functions/types to the supported jsonb equivalents.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update

Related Issue

Closes #1075

Changes Made

• Added a PostgreSQL provider fallback from json_agg() / json_build_object() and json casts to their jsonb equivalents.
• Updated RisingWave compatibility information and PostgreSQL provider documentation.
• Added an integration test covering the RisingWave json to jsonb fallback.

Testing

• I have tested this locally
• I have added/updated tests
• All existing tests pass

Checklist

• My code follows the project's code style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If I changed src/lib/db/providers/, I updated the matching docs/providers/ documentation and tests/integration/db/ tests in the same PR (provider triad)

Additional Notes

The fix is implemented through the existing PostgreSQL provider fallback mechanism, so PostgreSQL connections continue using the original query when the JSON constructs are supported.

[cevheri]

Thanks for picking this up. I can't merge it yet: the fix is no longer in the diff. The merge cleanup commit (3b2e31c) put postgres.ts back to main and removed your test, so the PR now changes only the two docs files, and they describe a fallback the code doesn't have. CI passed because it ran main's code.

To finish it:

• Restore the provider change on top of current main. Besides the function names, the NULL::json and '[]'::json casts in postgres.ts need the same treatment, since RisingWave refuses those too.
• Add a test to tests/integration/db/postgres-provider.test.ts that fails on main and covers describeObject() and describeObjects(). Please use the error RisingWave actually returns (recorded in compatibility.ts), not "type json does not exist".
• Update the README row and the caveat in compatibility.ts from your RisingWave run, and keep the parts that are still true (blank row counts and sizes, materialized views).

Small one: a tab replaced the "t" of "takes" in postgres.md. Please also update the PR description to match the final diff.