## Why
Remote credential masking previously fell back to unmasked execution whenever custom credential providers were configured.
## What changed
- Transport configured provider rules to the executor-local network proxy, where credential values and destination context are resolved.
- Advertise `credentialMaskingCustomProviders` and negotiate masking support. Keep the warning and unmasked fallback for executors that support only built-in providers.
- Discard provider rules when masking is disabled and omit their contents from debug output.
## Testing
Extend coverage for capability compatibility, provider configuration round trips, and fallback on older executors. Exercise configured-provider masking in remote request history and verify that masked placeholders are restored to real credentials in upstream request headers.
GitOrigin-RevId: b036f56900b343ddccc17b5293c82bc1f3426237