fix: support BigQuery UNNEST WITH OFFSET without AS alias (#6547)

[Zjianru]

Problem / 问题

BigQuery SQL SELECT * FROM UNNEST([10,20,30]) as numbers WITH OFFSET fails with ParserException because the parser unconditionally calls exprParser.expr() after WITH OFFSET, even when no AS alias is specified.

BigQuery SQL SELECT * FROM UNNEST([10,20,30]) as numbers WITH OFFSET 解析失败，抛出 ParserException。原因是解析器在 WITH OFFSET 后无条件调用 exprParser.expr()，即使没有 AS alias。

Root Cause / 根因

In SQLSelectParser.parseUnnestTableSource(), the code was:

lexer.nextIf(Token.AS);  // AS is optional
unnest.setOffset(this.exprParser.expr());  // always called — fails on ";"

When WITH OFFSET has no AS alias, the next token is ; (or EOF), and expr() cannot parse it.

在 SQLSelectParser.parseUnnestTableSource() 中，AS 是可选的，但 expr() 总是被调用。当 WITH OFFSET 后面没有 AS alias 时，下一个 token 是 ;（或 EOF），expr() 无法解析。

Fix / 修复

1. SQLSelectParser.java: Make the offset alias truly optional — only parse an alias expression when AS is present or a valid identifier follows.
2. SQLUnnestTableSource.java: Add boolean withOffset field to distinguish "has WITH OFFSET but no alias" from "no WITH OFFSET at all".
3. SQLASTOutputVisitor.java: Output WITH OFFSET (without AS) when withOffset is true but no alias expression exists.

Tests / 测试

Added 3 test cases to UnnestTest.java:

• test_unnest_with_offset_no_alias: WITH OFFSET without any alias (the reported bug)
• test_unnest_with_offset_as_alias: WITH OFFSET AS off (existing behavior, regression check)
• test_unnest_with_offset_implicit_alias: WITH OFFSET off (without AS keyword, but with alias)

All 4 tests pass (including the original test_0).

Closes #6547

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