Introduce snapshot testing for Expr::to_sexpr() in sexpr.rs

[coderabbitai]

Background

PR #259 extracted Expr::to_sexpr() into src/parser/ast/expr/sexpr.rs. The output is a text-based S-expression representation used in parser tests. Currently this output is validated only by inline string assertions; there are no snapshot tests to guard the rendering against regression.

Required work

Introduce snapshot testing for src/parser/ast/expr/sexpr.rs using the insta crate:

1. Add insta as a dev-dependency in Cargo.toml.
2. Write snapshot tests covering each Expr variant rendered by to_sexpr(), using insta::assert_snapshot!.
3. Commit the generated .snap files under src/parser/ast/expr/snapshots/ (or the default insta snapshot directory).
4. Confirm cargo test and cargo insta review pass cleanly.

Rationale

Snapshot tests make regressions in S-expression output immediately visible as diff noise in review, rather than requiring manual comparison of string literals scattered across test functions.

References

• PR Refactor large parser modules into smaller components under 400 lines (#223) #259: Refactor large parser modules into smaller components under 400 lines (#223) #259
• Raised as out-of-scope in review comment: Refactor large parser modules into smaller components under 400 lines (#223) #259
• Requested by @leynos