fix(router): backport unknown worker target validation to 1.5 (#14858)

[jthomson04]

Summary

Backport #14858 to release/1.5.0. An unknown effective nvext worker target now returns InvalidArgument (HTTP 400) at initial selection. The error identifies the field and rejected worker ID.

Details

• Cherry-pick of 5e21f9c6181ad42ba4fb8a3e20bf7c2b62631724.
• Covers built-in and KV routing, prefill, and route previews. Preserve field precedence, LoRA selection, affinity handling, and service-error handling after selection.
• Resolve one append conflict in routing_host/tests.rs by retaining both the release branch's shutdown/migration regression and the incoming explicit-worker tests. Use instance_ids().contains() in one test because Client::is_instance_live() is absent on this release. Production changes are unchanged from the source PR.

Where should the reviewer start?

lib/llm/src/kv_router/routing_host.rs for target validation, then prefill_router/mod.rs for preview error propagation and routing_host/tests.rs for regression coverage.

Related Issues

• Relates to fix(router): reject unknown explicit worker targets #14858 and DYN-4446.

Validation

Run locally on the release backport with default test parallelism and debug info disabled:

• cargo test -p dynamo-llm --no-default-features --lib kv_router::routing_host::tests — 48 passed, including the retained shutdown/migration test.
• cargo test -p dynamo-llm --no-default-features --lib kv_router::prefill_router:: — 33 passed.
• cargo test -p dynamo-llm --no-default-features --lib session_affinity:: — 37 passed.
• cargo test -p dynamo-llm --no-default-features --lib http::service::openai::tests:: — 130 passed.
• cargo fmt --all -- --check and git diff --check HEAD^ HEAD — passed.

No GPU/vLLM end-to-end test was run.