fix(deparser): MergeWhenClause support and SecLabelStmt qualified object names

[pyramation]

Summary

Two pgsql-deparser bugs found while fuzzing constructive-db's schema transform:

1. Top-level MERGE could not deparse at all — MergeStmt visited its mergeWhenClauses, but there was no MergeWhenClause visitor, so every MERGE failed with Deparser does not handle node type: MergeWhenClause. Added the visitor covering:

   • MERGE_WHEN_MATCHED / NOT MATCHED [BY TARGET] / NOT MATCHED BY SOURCE
   • AND <condition>
   • CMD_UPDATE (UPDATE SET via the same ResTarget/update context as UpdateStmt), CMD_INSERT (columns, OVERRIDING {USER|SYSTEM} VALUE, VALUES (...) / DEFAULT VALUES), CMD_DELETE, CMD_NOTHING (DO NOTHING)
   • also added MergeStmt.returningList handling (MERGE ... RETURNING, PG17)

2. SecLabelStmt emitted invalid qualified names — SECURITY LABEL ... ON COLUMN a.b.c deparsed to ON COLUMN "a", b, c because the object List was visited generically (comma-joined). Now dot-joined, matching CommentStmt's existing handling.

Fixtures added via the kitchen-sink pipeline (__fixtures__/kitchen-sink/misc/merge-seclabel.sql, 8 statements round-tripped in both plain and pretty modes). Full deparser suite passes (713 tests).

Link to Devin session: https://app.devin.ai/sessions/eeb8deba0c04475091df703c88877302
Requested by: @pyramation

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