feat: add mutable visitor trait for traversal of AST

[scintillavoy]

Partially addresses #137

This PR implements the mutable version of the AST visitor (ExprVisitorMut and walk_expr_mut) to allow in-place mutations of the AST.

To support mutating extension nodes, I added children_mut() to the ExtensionExpr trait. Because the extension holds the inner expression in an Arc<dyn ExtensionExpr>, walk_expr_mut attempts to unwrap it using Arc::get_mut. Currently, if Arc::get_mut fails (meaning there are multiple references to the shared Arc), the walker falls back to treating the extension as a leaf node and skips traversing its children.

I'm not entirely sure if this fallback behavior is the most reasonable approach. I would love to hear your thoughts on whether we should keep it this way, or perhaps delegate the decision to the visitor itself (e.g., by adding an optional visit_shared_extension method to the ExprVisitorMut trait so implementers can choose whether to skip, halt, or throw a custom error).

As discussed in the issue, I implemented this manually without using macros for now. I realized it would be more complicated than expected to use a macro to deduplicate the code, since Expr::Extension must be handled differently depending on whether it is immutable or mutable.

[codecov]

Codecov Report

❌ Patch coverage is 83.69565% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.53%. Comparing base (c4bebc8) to head (58a425e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/util/visitor.rs	83.69%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
- Coverage   98.93%   98.53%   -0.41%     
==========================================
  Files          15       15              
  Lines        6560     6876     +316     
==========================================
+ Hits         6490     6775     +285     
- Misses         70      101      +31     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[scintillavoy]

❌ Patch coverage is 80.95238% with 32 lines in your changes missing coverage. Please review.

Tried to open the report, but it returns a "400 Bad Request" error when I click on a src/util/visitor.rs file to see the exact missed lines.

[evenyag]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 156c7d09c4

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[evenyag]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 101c540040

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[evenyag]

LGTM