[Feat][Router]: allow healthchecks to be disabled by models

[max-wittig]

We want to keep the global healthcheck enabled and just disable it for some models that might involve cost per request

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[gemini-code-assist]

Code Review

This pull request introduces the ability to disable health checks for specific static backends via a new --static-healthcheck-disabled CLI flag and a healthcheck_disabled YAML configuration field. The review feedback identifies a missing null check when parsing this new field during dynamic reconfiguration and a potential TypeError in the health check loop if model types are not provided.