feat(plpgsql-parser): automatically compute return info for correct RETURN handling

[pyramation]

Summary

The plpgsql-parser now automatically extracts return type information from CreateFunctionStmt and passes it to the PL/pgSQL deparser. This ensures correct RETURN statement handling without users needing to manually provide return context.

Behavior:

• SETOF functions: bare RETURN; stays as RETURN;
• Scalar functions: empty RETURN; becomes RETURN NULL;
• Void functions: bare RETURN; stays as RETURN;
• Trigger functions: handled correctly
• OUT parameter functions: bare RETURN; stays as RETURN;

Updates since last revision

Documentation updates to clarify the separation between packages:

• plpgsql-deparser README: Now clearly states it's a "body-only" deparser and directs users to plpgsql-parser for full SQL + PL/pgSQL deparsing
• plpgsql-parser README: Updated to emphasize it handles "heterogeneous parsing and deparsing" - the full pipeline for complete CREATE FUNCTION statements

Previous updates:

• round-trip.test.ts: New test file that runs all 190 generated fixtures through the full plpgsql-parser.parse() → plpgsql-parser.deparseSync() → reparse → compare AST pipeline
• This ensures regressions in the automatic return info handling will be caught by fixtures (not just explicit unit tests)
• All 190 fixtures pass round-trip testing through plpgsql-parser

Review & Testing Checklist for Human

• Verify the if (plpgsqlFunc) guard in deparse.ts doesn't silently skip function body stitching - if dehydrated.plpgsql_funcs?.[0]?.PLpgSQL_function is undefined when it shouldn't be, the function body won't be updated
• Test with a real PL/pgSQL function that has an empty RETURN in a scalar context to confirm it outputs RETURN NULL
• Verify the cleanSqlTree function in round-trip tests strips only location metadata and doesn't mask real AST differences
• Review README changes for accuracy - ensure the package separation guidance is clear and correct

Recommended test plan: Run pnpm test in packages/plpgsql-parser to verify all 92 tests pass (including the new round-trip tests that exercise all 190 fixtures).

Notes

• This change makes the ReturnInfo parameter an internal implementation detail - users of plpgsql-parser no longer need to know about it
• Link to Devin run: https://app.devin.ai/sessions/8e1c971e9b194cd9a7dda034c89bd74b
• Requested by: @pyramation

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