fix LSP highlights wrong argument when using a call as argument #2500

[asukaminato0721]

Summary

Fixes #2500

active-argument inference counts only top-level argument separators, ignoring commas inside nested calls, tuples/lists/braces, strings, and comments.

Test Plan

add test

[Copilot]

Pull request overview

This PR fixes LSP signature help’s active-argument inference by counting only top-level argument separators (commas), ignoring commas that occur inside nested calls/containers, strings, and comments—addressing Issue #2500 where nested commas caused the wrong parameter to be highlighted.

Changes:

• Replace naive comma counting with a lightweight depth-aware scanner that skips Python string literals and line comments.
• Add regression tests covering nested calls, tuple literals, and commas inside strings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/lsp/wasm/signature_help.rs	Updates active-argument inference to count only top-level commas, skipping nested delimiters, strings, and comments.
pyrefly/lib/test/lsp/signature_help.rs	Adds a test ensuring nested commas don’t affect active_parameter selection.

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[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅