feat: As a user, I want to healthcheck add mtls support

### Description

When the upstream needs to use mtls, healthcheck does not reuse ssl certificates, so that all health checks return http 400 code.

Hope to be able to support such scenarios

Start at the healthcheck entry point and inspect how upstream SSL certificates are handled. Reproduce the reported HTTP 400 response with an mTLS upstream and compare it with a non-mTLS check; done means health checks succeed when the upstream requires mTLS, with regression coverage.

Written by the indexing model from the issue text.