rustdoc: Since 1.87 we permit shebangs in doctests that have a `main` fn

[fmease]

Note

This topic was already extensively discussed in #t-rustdoc/meetings > 2025-09-08 @ 💬. Verdict is still unclear, namely whether to simply accept it as an accidental feature (would need FCP after the fact with motivation) or revert (wouldn't need an FCP, might need a crater run).

Since PR #138104 (stabilized in 1.87), we accidentally permit rustdoc file.rs --test where file.rs:

//! ```
//! #!XYZ XYZ XYZ XYZ XYZ
//! fn main() {}
//! ```