Preserve nested PostgreSQL function defaults

[yzxcj797]

Summary

Importing a PostgreSQL table with a default expression containing nested function calls discarded the arguments. For example, the issue's pg_dump default became:

substring(, 1, 6)

The shared SQL AST serializer now recursively renders function arguments and PostgreSQL ::type casts. PostgreSQL function defaults use that serializer instead of a one-level local conversion, preserving expressions such as:

substring(md5(random()::TEXT), 1, 6)

Fixes #431.

Testing

• npm test — 1/1 regression test passes
• npm run lint — passes with zero warnings
• npm run build — passes (existing lottie-web eval and large-chunk warnings remain)
• npx prettier --check on changed files — passes
• git diff --check — passes

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