Harden Apps Script forwarder validation

[poulcarlsen53]

Summary

This tightens the Apps Script relay script without changing the tunnel protocol.

It rejects obvious non-tunnel POST bodies before spending UrlFetchApp quota, expands the relay loop guard to cover script.googleusercontent.com variants, and exposes a little more safe deployment metadata from doGet.

Why

A mispointed RELAY_URLS value or public probe can waste daily Apps Script quota and produce confusing non-batch responses in the client. This keeps the script as a simple encrypted forwarder, but makes the edges safer and easier to diagnose.

Notes

I intentionally did not add a custom UrlFetchApp.fetch timeout option here because Apps Script does not document a supported per-request timeout parameter for UrlFetchApp. The script still uses followRedirects: false and muteHttpExceptions: true, and still throws when every upstream fails so clients treat that as an endpoint failure instead of trying to decode an error string.

Validation

• go test -count=1 ./apps_script ./internal/carrier
• go test -count=1 ./...