feat: add default_value_ast column for SQL expression validation

[pyramation]

Summary

Adds default_value_ast jsonb column to collections_public.field table and marks default_value with the @sqlExpression smart comment for the Graphile SQL expression validator plugin.

This is the upstream change for constructive-db PR #178. The AST column will store the parsed representation of SQL default value expressions, with AST being the source of truth for validation.

Related to constructive-io/constructive-planning#476.

Review & Testing Checklist for Human

This is a low-risk schema addition (nullable column, no data migration):

• Verify coordination with constructive-db: PR #178 in constructive-db has the same changes in extensions/@pgpm/db-meta-schema/. After merging this PR, constructive-db should pull from upstream to stay in sync
• Test deployment: Run the deploy script against a test database to verify the ALTER TABLE and COMMENT statements execute correctly

Notes

The @sqlExpression smart comment works with the graphile-sql-expression-validator plugin (merged in constructive PR #543). Without the plugin loaded, the comment is inert.

Link to Devin run: https://app.devin.ai/sessions/7655646c517d4c74b0f7b827a102d494
Requested by: Dan Lynch (@pyramation)

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