Invalid integer literals are accepted

Integer literals with suffix starting with `e` are accepted by `proc_macro2` but not by `proc_macro` or Rust compiler.

```Rust
let _: TokenStream = "0einvalidSuffix".parse().unwrap();
```
This parses with `proc_macro2` but panics with `proc_macro`.

Such literals are also rejected by Rust compiler: [playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=1c68cdc180ae546a281af81d4ff27870).

According to Rust reference [INTEGER_LITERAL](https://doc.rust-lang.org/stable/reference/tokens.html#railroad-INTEGER_LITERAL) must end with [SUFFIX_NO_E](https://doc.rust-lang.org/stable/reference/tokens.html#railroad-SUFFIX_NO_E).

Reproduce the issue with the shown `TokenStream` `.parse()` entry point and compare its result with `proc_macro` and the Rust compiler. Trace how integer literals and suffixes are parsed, then verify that `0einvalidSuffix` is rejected consistently with the Rust reference grammar.

Written by the indexing model from the issue text.