[ty] Avoid AST load for callable description names (#25728)
## Summary
`CallableDescription` loaded the parsed module only to read the
enclosing class name when formatting `Class.method` for function and
bound-method diagnostics.
This now recovers the enclosing class from the semantic index and
formats the name from the original class type metadata, avoiding that
direct AST node lookup while preserving the same display text.
