caddytest: close temp config file handle in validateTestPrerequisites

[vvdvortsova]

I'm not a native English speaker, so I used AI to help translate this PR description. The code and analysis are my own.

What

Fix resource leak in validateTestPrerequisites.
Fix for issue #7833.

validateTestPrerequisites in caddytest/caddytest.go created a temp file via os.CreateTemp and only removed it from disk in t.Cleanup (os.Remove(f.Name())), never closing the file descriptor including on the early return err if writing the config failed. Added defer f.Close() right after the file is created.

Testing
go build ./caddytest/...
go vet ./caddytest/...

[CLAassistant]

All committers have signed the CLA.

[steadytao]

This should close the temp file before starting Caddy from it. With the current defer, the handle stays open until ValidateTestPrerequisites returns so Caddy may still read the config while the writer handle is open. Closing immediately after the fmt.Fprintf succeeds would be cleaner. We also need a signed CLA and an AI assistance disclosure, please.

[vvdvortsova]

This should close the temp file before starting Caddy from it. With the current defer, the handle stays open until ValidateTestPrerequisites returns so Caddy may still read the config while the writer handle is open. Closing immediately after the fmt.Fprintf succeeds would be cleaner. We also need a signed CLA and an AI assistance disclosure, please.

I added signed CLA and added f.Close() after fmt.Fprintf call. I used AI assistance (Claude) to write this PR. I'm not a native English speaker.