test: cover workspace symbol aliases

[mawxcodehub]

Summary

• adds regression coverage for workspace symbols backed by frontmatter aliases
• checks both the empty-query symbol list and a filtered alias query
• keeps the existing filename symbol behavior covered in the same fixture

Closes #263.

Verification

cargo test workspace_symbols_include_frontmatter_aliases was not runnable in my local Windows environment because cargo is not installed there. The change is test-only and git diff --check passes locally.