## Why
Model provider configuration could override the residency header required by
`enforce_residency`, so provider-backed requests did not reliably honor the
managed setting.
## What changed
- Apply the managed residency header after building a provider, making it
authoritative for model requests and model discovery.
- Warn when a provider configures the residency header through
`http_headers` or `env_http_headers`, while preserving the original provider
configuration and unrelated headers.
- Cover HTTP and WebSocket requests, model discovery, and case-insensitive
header detection with targeted tests.
GitOrigin-RevId: 461ef8989d5bd06ffdea694aee4265e392e07590
