feat(@angular/build): add prerenderFormat option to prerender routes as <route>.html

[JohannesHoppe]

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• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Prerendering always writes a route to <route>/index.html. On static hosts, this forces a choice between clean URLs and no redirects. You can't have both:

• Clean URLs, but redirects: links use /<route>. Every direct request (search engines, bookmarks, shared links) is first redirected (301/308) to /<route>/, and the Angular router then removes the trailing slash again.
• No redirects, but trailing slashes everywhere: links have to use /<route>/, and the app needs TrailingSlashPathLocationStrategy to keep the slash in the address bar.

Issue Number: #29173

What is the new behavior?

A new application builder option prerenderFormat makes both possible: clean URLs without a trailing slash, served directly with a status code 200.

• "directory" (default): /foo/bar is written to foo/bar/index.html, unchanged.
• "file": /foo/bar is written to foo/bar.html, so hosts that serve <route>.html for /<route> respond to /foo/bar without a redirect.

Mirrors Astro's build.format ('directory' | 'file'); Next.js, SvelteKit and Hugo offer the same choice via trailingSlash/uglyURLs.

• The root route of the application and of each locale stays index.html, so / and /<locale>/ keep being served by the host's directory index.
• "file" requires a build without a server. The @angular/ssr runtime looks up prerendered pages as <route>/index.html (AngularServerApp.buildServerAssetPathFromRequest, CommonEngine.retrieveSSGPage), and with a server there is no redirect to avoid, since the generated server.ts serves static files with redirect: false.
• A route ending in index (for example /index or /Index) is reported as an error with "file", because its file would be index.html of the parent path.
• The dev server does not prerender. When it switches a prerendering build to on-demand SSR, it resets the option. Otherwise, ng serve reports the same error as ng build.
• It is a top-level option rather than part of prerender, because prerender is not considered when outputMode is set.

As suggested by @SanderElias in the issue, the option description states that not all hosting services support this. Happy to rename the option if you prefer a different name.

Does this PR introduce a breaking change?

• Yes
• No

Other information

As a stopgap, I built a builder (@angular-schule/prerender-format) that wraps @angular/build:application. It only works by replacing the internal prerenderPages() export of @angular/build at runtime to rename the output files, which is fragile and can break with any internal refactoring. A built-in option is the clean solution.

The option description in schema.json feeds the ng build reference on angular.dev. If this lands, I'm happy to follow up with a short section in the SSR guide ("Generate a fully static application") in angular/angular.