Create is_sitemap() conditional tag

[MadtownLems]

As people continue expanding and customizing Core's XML sitemaps, I believe an is_sitemap() conditional tag would be very helpful.

In the short term, its existence would be very useful for getting around the bug explained in #51542 (https://core.trac.wordpress.org/ticket/51542), but surely developers will find other ways to make use of it and it seems like a natural fit to be a conditional tag.

[RavanH]

The is_sitemap() conditional is now provided by ​https://wordpress.org/plugins/xml-sitemaps-manager/ :)

This patch introduces is_sitemap() as a conditional query tag and adds the $is_sitemap property to WP_Query, following the established pattern of is_robots() and is_favicon().

Trac ticket: https://core.trac.wordpress.org/ticket/51543

## Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): Claude Sonnet 4.6
Used for: Initial implementation scaffolding. All code was reviewed, adjusted, and tested by me.

@mukeshpanchal27 to make reviewing easier i split the second trac ticket to a own PR, see: ​https://github.com/WordPress/wordpress-develop/pull/12150

✅ New code passes PHPStan rule level 10 with the bleeding edge ruleset.

@westonruter thanks to @nimesh-xecurify we have now tests as well for this PR :)

[mukesh27]

@westonruter This one is ready for commit

[westonruter]

In 62664:

Sitemaps: Add the is_sitemap() conditional tag.

Introduce is_sitemap() as a conditional tag, mirroring the existing is_robots() and is_favicon() tags, and add the corresponding $is_sitemap property to WP_Query. The flag is set whenever the sitemap query variable is present, and a sitemap request is now excluded from being treated as the home page.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/12142.
Follow-up to r47018, r48072.

Props masteradhoc, nimeshatxecurify, westonruter, mukesh27, madtownlems, ravanh, cybr, swissspidy.
See #51117.
Fixes #51542, #51543.

[westonruter]

Note: The is_sitemap() function does not return true for sitemap stylesheet requests, e.g. /wp-sitemap-index.xsl.

This XSL stylesheet is referenced in the sitemap XML:

<?xml-stylesheet type="text/xsl" href="http://localhost:8000/wp-sitemap-index.xsl" ?>

I think we should remove the sitemap stylesheet from core, however.

Chrome is ​deprecating and removing XSLT, and other browsers are doing the same.