feat(server): support multiple hosts in __VITE_ADDITIONAL_SERVER_ALLOWED_HOSTS

[ctate]

Allows __VITE_ADDITIONAL_SERVER_ALLOWED_HOSTS environment variable to accept multiple comma-separated hosts instead of just a single host.

# Before: single host only
__VITE_ADDITIONAL_SERVER_ALLOWED_HOSTS=example.com

# After: multiple hosts supported
__VITE_ADDITIONAL_SERVER_ALLOWED_HOSTS=example.com,test.com,dev.example.org

• Splits the environment variable value by commas
• Trims whitespace from each host
• Filters out empty entries
• Fully backwards compatible with single host values
• Updated documentation (docs/config/server-options.md)
• Included tests (6 new unit tests in config.spec.ts)

[bluwy]

The change looks good to me. Can you share a bit of your usecase here?

[ctate]

@bluwy Thanks for reviewing!

The use case is cloud-based development environments that can generate multiple hostnames for a single dev session. These platforms sometimes rotate or assign multiple preview URLs that all need to be allowed.

With the previous implementation only supporting a single host, devs would hit the "This host is not allowed" error and have to manually edit vite.config.js. This isn't ideal because those environment-specific changes could accidentally get committed and pushed to the repo.

By parsing the env var as comma-separated values, cloud IDEs can transparently configure all their hosts via the environment - no config file changes needed, nothing to accidentally commit.

[bluwy]

I just realized a similar PR was opened before: #19746. The discussion there was to have proper comma parsing, though for me I'm not sure we need it for now.