`astro build` fails unexpectedly with `build.format` set to `preserve`

### Astro Info ```block Astro v7.2.0 Vite v8.2.1 Node v26.7.0 System macOS (arm64) Package Manager npm Output static Adapter none Integrations none ``` ### If this issue only occurs in one browser, which browser is a problem? _No response_ ### Describe the Bug After upgrading astro from version `5.17.1` to version `7.2.0` for a project, I became unable to build astro pages via `astro build` whose astro files: 1. sit within `/src/pages/[format]/[lang]/` 2. had the following content within their frontmatter: ``` export async function getStaticPaths() { return [ { params: { format: 'first', lang: 'en' } }, { params: { format: 'second', lang: 'en' } }, { params: { format: 'third', lang: 'en' } }, { params: { format: 'first', lang: 'de' } }, { params: { format: 'second', lang: 'de' } }, { params: { format: 'third', lang: 'de' } }, ]; } const {format, lang} = Astro.params; ``` When e.g. trying to `astro build` the page `/src/pages/[format]/[lang]/welcome.astro`, I got a build error saying: `first/en/welcome.html13:58:55 [ERROR] TypeError: Missing parameter: format` ... indicating that while the path parameter `format` of the page `/src/pages/[format]/[lang]/welcome.astro` could be properly resolved, it could not be recognized by the `astro build` cycle. After hours of console-logging and seeing that the path parameters are properly resolved and available while the page was being built, I found that the problem was that I set the [build.format](https://docs.astro.build/en/reference/configuration-reference/#buildformat) option in the astro config file to `preserve`. When uncommenting that, the build succeeded as in version `5.17.1`. I further investigated on this and indeed setting the [build.format](https://docs.astro.build/en/reference/configuration-reference/#buildformat) option to `directory` (which is the default value hence equivalent of omission tested above) indeed does the trick. That feels like a bug to me, especially because it did not happen in version `5.17.1`; so I wondered if this change is intentional ? If so, it would be very helpful if the [SSG docs](https://docs.astro.build/en/guides/routing/#static-ssg-mode) could e.g. include a note about an incompatibility with the [build.format](https://docs.astro.build/en/reference/configuration-reference/#buildformat) setting. ### What's the expected result? Pages should be generated normally as in version `5.17.1`. In the linked example, try simply changing the value of the `build.format` config from `preserve` to `directory` and vice-versa in the projects' `astro.config.mjs` file, then re-run `npm run build` to try to rebuild the project and you will see what I mean. ### Link to Minimal Reproducible Example https://stackblitz.com/edit/github-hdpeidlx?file=astro.config.mjs ### Participation - [ ] I am willing to submit a pull request for this issue.