fix(codegen): only add mapping names for `PrivateIdentifier`s which have changed (#25958)

A `PrivateIdentifier`'s span includes the leading `#`, but the `name` the AST holds does not.

The source map name was read from the whole span and then compared to `name` from AST, so the check which omits the mapping's `name` when it wasn't renamed compared `#foo` against `foo`. It never matched, so a named mapping was created for _every_ private identifier, renamed or not.

This PR fixes that in both Rust `oxc_codegen` and JS `oxc-codegen`, by comparing correctly including the `#`.

This also makes mapping names correct when a normal `Identifier` has been replaced with a `PrivateIdentifier` (`this.foo` -> `this.#foo)`, or vice-versa (`this.#foo` -> `this.foo`). New tests cover both directions.