Warning for absence of `doc-scrape-examples = true` should respect `--deny warnings`

Tracking issue: #88791

RUSTDOCFLAGS='-D warnings' cargo +nightly doc --all-features -Zunstable-options -Zrustdoc-scrape-examples

I expected to see this happen:

A hard error, and cargo doc exits with a non-zero code, so we can check it in CI.

error: Rustdoc did not scrape the following examples because they require dev-dependencies: xxx, yyy, zzz
    If you want Rustdoc to scrape these examples, then add `doc-scrape-examples = true`
    to the [[example]] target configuration of at least one example.

Instead, this happened:

Still a warning even we have passed -D warnings.

warning: Rustdoc did not scrape the following examples because they require dev-dependencies: xxx, yyy, zzz
    If you want Rustdoc to scrape these examples, then add `doc-scrape-examples = true`
    to the [[example]] target configuration of at least one example.

Meta

rustc +nightly --version --verbose:

rustc 1.93.0-nightly (adaa83897 2025-10-27)
binary: rustc
commit-hash: adaa838976ff99a4f0661136322f64cb466b58a0
commit-date: 2025-10-27
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.3

Start by reproducing the issue with the provided RUSTDOCFLAGS and cargo doc command, then trace how rustdoc emits the doc-scrape-examples diagnostic and applies warning levels. Done means -D warnings turns this warning into a hard error and the command exits non-zero, with coverage for the behavior.