fix: recover RETURN NEXT variable lost by libpg-query + CoalesceExpr/RangeFunction test fixtures

[pyramation]

UPDATE: don't do this, inferReturnNextVar is attempt, not actually correct in all cases.

we need to wait for an upstream PR in libpg-query

Summary

Three commits, telling the story: broken test → fix → pass.

Commit 1: test: add CoalesceExpr and RangeFunction round-trip fixtures

9 test fixtures (Tests 38–46) confirming pgsql-parser correctly handles CoalesceExpr in FuncCall args and FuncCall in RangeFunction/FROM clauses. All round-trip correctly.

Commit 2: test: demonstrate RETURN NEXT retvarno loss (proves the bug)

Adds fixtures 47-48 and snapshot tests that prove the bug exists: libpg-query does not serialize retvarno for PLpgSQL_stmt_return_next, so RETURN NEXT v_entry deparses as bare RETURN NEXT;. The snapshots capture the broken output:

RETURN NEXT;      -- should be: RETURN NEXT v_entry;
RETURN NEXT;      -- should be: RETURN NEXT v_val;

Fixtures are added to KNOWN_FAILING_FIXTURES in this commit.

Commit 3: fix: recover RETURN NEXT variable when retvarno is lost

Fixes the deparser to recover the variable:

• Thread enclosingForVarName through all FOR loop contexts
• inferReturnNextVar() recovers from: (1) enclosing FOR loop variable, (2) single user-declared variable
• Safety: only infers when returnInfo is provided with kind='setof' — OUT-param functions keep bare RETURN NEXT;
• extractReturnInfo() in test-utils extracts return type from CREATE FUNCTION SQL for round-trip tests
• Removes fixtures 47-48 from KNOWN_FAILING_FIXTURES — they now pass
• Updates snapshots to show recovered output: RETURN NEXT v_entry;, RETURN NEXT v_val;

All 236 fixtures round-trip. 95 tests pass. 65 snapshots match.

Review & Testing Checklist for Human

• Check commit 2 in isolation — its tests assert the broken output (RETURN NEXT; without variable). This proves the bug.
• Check commit 3 — same test functions now assert the fixed output (RETURN NEXT v_entry). Snapshots updated.
• Verify RETURN NEXT; stays bare for OUT-param functions (test: "should leave RETURN NEXT bare for OUT-param functions")
• Verify inference does NOT happen when returnInfo is not provided (test: "should leave RETURN NEXT bare when returnInfo is not provided")

Notes

The extractReturnInfo() helper only returns returnInfo for SETOF and OUT-param functions. Other return types (scalar, void, trigger) intentionally return undefined to avoid changing existing RETURN statement behaviour.

Link to Devin session: https://app.devin.ai/sessions/c0494871633d4beb91f1e16e53c776d1
Requested by: @pyramation

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