## Summary of changes
Prevented compiler-generated async/closure implementation types (e.g.
`+<DoAsyncWork>d__3`, `+<>c__DisplayClass`…) from leaking into Live
Debugger symbols by sanitizing the implicit `this` argument type for
closure scopes to the user-declared declaring type.

## Reason for change
Customers see confusing symbol entries containing <>/<…>d__… and other
compiler artifacts in symbol search results. These are valid CLR
metadata names but are not meaningful to users and clutter the Live
Debugger UI.

## Test coverage
Updated existing SymbolExtractorTest Verify approval snapshots to
reflect the sanitized this type in closure scopes.

## Follow-ups
If customers still see compiler-generated names from other symbol fields
(e.g., local variable), we can add additional targeted sanitization
rules with explicit replacements.