Fix false duplicate warning for sitemap with generateSitemaps

[SahilMotiramani]

What does this change?

When app/sitemap.ts uses generateSitemaps(), Next.js only generates
/sitemap/[id].xml routes and does not generate a root /sitemap.xml.

However, in next dev, the app route matcher always reserved /sitemap.xml
for sitemap.ts, which caused a false duplicate route warning when users
defined app/sitemap.xml/route.ts for a sitemap index.

This PR updates the dev app route matcher to skip generating the single
sitemap.xml route for sitemap entries and adds a regression test.

Why is this safe?

• Dev-only change
• No production behavior changes
• No breaking changes
• Covered by a new test

Reproduction

See issue #87314.

[SahilMotiramani]

This PR is from a first-time contributor.
The build-native-windows job failed due to runner validation.
Could a maintainer please rerun or approve the workflow?
Thanks!

[SahilMotiramani]

@eps1lon would like to request a review, if any changes required please tell else please merge

[SahilMotiramani]

@feedthejim @unstubbable @timneutkens
I would like to request do a review on my pull and if you find it correct please merge or please inform the changes😊