fix: strip cache_control.scope from content blocks

[Alex-dev-spec]

Problem

Claude Code now sends cache_control.scope in content blocks, which the Anthropic API rejects with:

system.N.cache_control.ephemeral.scope: Extra inputs are not permitted

This causes 400 errors on every request that includes system prompt caching when using a recent version of Claude Code.

Fix

Strip cache_control.scope in processContentArray, mirroring the existing logic that already strips cache_control.ttl for the same reason.

if (item.cache_control.scope) {
  delete item.cache_control.scope;
  Logger.debug('Removed scope from cache_control');
}

Testing

Deployed and tested on a production GCP proxy instance for 2 days. No 400 errors related to cache_control fields after the fix. Behaviour is identical to the ttl stripping already present in the codebase.