Expression支持获取它的List<Token>

背景
业务上想基于Expression 对应的 List 做解析和展示，通过 `ExpressionLexer` 解析出来的 token 列表跟 `OptimizeCodeGenerator` 解析出来的 token 列表不太一致。

是否可以将 OptimizeCodeGenerator 解析后的token 列表赋值给Expression,Expression提供 getTokens 方法，返回只读列表。 这样也能复用Expression已有的缓存。

考虑不周之处，请指正。

Start by locating Expression, ExpressionLexer, and OptimizeCodeGenerator, then trace how token parsing and Expression's existing cache are handled. Determine how the optimized token list can be stored and exposed through a read-only getTokens method. Done means callers can obtain the optimized tokens consistently without modifying the returned list.

Written by the indexing model from the issue text.