fix(nextjs): restore the user config when the override fails

[Om-singhaI]

overrideNextConfig renames the user's Next config before anything that can throw, and
bin/build.ts called it outside the try whose finally restores it. Any failure after the
rename left the project holding next.config.original.<ext> and no config at all, and a later
run does not recover it: loadConfig then reports the default next.config.js, so
restoreNextConfig looks for a backup under the wrong extension and returns early.

Cloud Build workspaces are ephemeral, but the adapter also runs locally through
npm exec apphosting-adapter-nextjs-build, where this mutates the developer's tree and does
not put it back.

The extension check and the config content now happen before the rename, the original is
restored if the write fails, and the override and its validation moved inside the try.

The new test covers the path that throws before the rename. The restore in the catch is
covered by inspection only, since making writeFile fail after a successful rename in the
same directory is not something I could drive from a unit test.

#684 shipped .mts support, which was the other half of #680.

Fixes #680

[gemini-code-assist]

Code Review

This pull request adds support for the .mts file extension in Next.js configuration overrides and improves error handling during the override process. Specifically, validation of the file extension is now performed before renaming the original configuration file, and a fallback mechanism has been added to restore the original configuration if the override process fails. Unit tests have also been added to verify these changes. I have no feedback to provide.

[Om-singhaI]

#684 covered the .mts half of #680, so I have rescoped this to item 2 of that issue, the rename that is not undone. It is independent of the extension: a writeFile failure strands a .ts project the same way, and a later run cannot recover it because restoreNextConfig then looks for the backup under the default next.config.js extension.

The branch is rebuilt on current main and shares no lines with #684. The extension check moved above the rename, the original is restored if the write fails, and the override and its validation moved inside the try whose finally already restores.