Fix PostgreSQL reserved keyword projection aliases

[terrymanu]

No description provided.

[terrymanu]

Result

Review Result: Mergeable

The PostgreSQL parser now accepts SELECT 1 AS desc and preserves desc as the AST alias. This is a local code review only; CI was not reviewed.

Evidence

• Explicit AS aliases now use colLabel; the Visitor handles both the new and existing alias paths.
• select_keyword moved from a negative case to a positive case that asserts the AST alias.
• PostgreSQL Parser IT: all 1,180 positive and 3 negative cases passed.
• Spotless passed; Checkstyle reported no violations.

Coverage

• Standalone local change based on 8695937f09a; all five changed files were reviewed.
• Grammar, Visitor, test assertions, and impact were checked. openGauss has an independent grammar and was not changed.
• No blockers found. The full repository build and CI were not run.