fix: skip unhealthy workers in proxy GET endpoints

[Bevisy]

Purpose

Fixes #135 — /v1/models (and other proxy GET endpoints) returns 500 when any worker is unhealthy.

select_first_worker returned the first registered worker without checking health state. worker_registry.get_all() returns all workers including unhealthy ones, so when the first worker was temporarily down, proxy_get_request proxied to a dead worker and returned 500. This affected /v1/models, /get_server_info, and /get_model_info — all of which route through proxy_get_request → select_first_worker.

Root-cause fix in the shared helper: filter for a healthy worker (is_healthy()) before returning; fall back to the existing error path when none are healthy.

Test Plan

New unit tests in routers::http::router::tests:

• test_select_first_worker_skips_unhealthy — marks worker1 unhealthy, asserts worker2 is selected
• test_select_first_worker_all_unhealthy — marks all unhealthy, asserts error

Test Result

running 3 tests
test routers::http::router::tests::test_select_first_worker_regular ... ok
test routers::http::router::tests::test_select_first_worker_skips_unhealthy ... ok
test routers::http::router::tests::test_select_first_worker_all_unhealthy ... ok

test result: ok. 3 passed; 0 failed

Built with cargo test --lib routers::http::router::tests::test_select_first_worker (rust:latest container, aarch64).