fix(parser): allow reserved keywords as aliases after AS

[VioletQwQ-0]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #24410

What this PR does / why we need it:

Allows reserved keywords (ROWS, CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, LOCALTIMESTAMP) to be used as column aliases after AS, matching MySQL behavior.

Before this fix:

SELECT NOW() AS current_time;  -- parser error near "current_time"
SELECT COUNT(*) AS rows FROM t; -- parser error near "rows"

After:

SELECT NOW() AS current_time;  -- works
SELECT COUNT(*) AS rows FROM t; -- works

Implementation

Lexer-level fix in mysql_lexer.go: track lastToken and convert the 5 reserved keyword tokens to ID when they immediately follow AS. No grammar changes needed.

Scope

This PR addresses sub-issue 1 of #24410 (reserved word aliases). Sub-issues 2 and 3 (TIMESTAMPDIFF units and aggregate-in-TIMESTAMPDIFF) were already fixed by #23161 (Dec 2025).

Test plan

• Parser regression tests (6 alias cases + 3 regression cases)
• End-to-end MO instance verification
• Full parser test suite passes
• Existing ROWS (window functions) and CURRENT_TIME() (function call) behavior unchanged

🤖 Generated with Claude Code

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[XuPeng-SH]

Looks good to me. I pulled the latest head, re-checked the lexer change against the current main base, and did not find a blocker. The allowlist is intentionally narrow, the AS-context rewrite only applies to the five reserved tokens needed for issue #24410, and it does not interfere with their original keyword uses like CURRENT_TIME() or ROWS in window frames. The added parser tests and the parser package test suite both look good for this fix.