refactor(miles): drop unused router engine metadata

[TianyeGGBond]

Context

This PR is intentionally separate from the health-check admission fix so reviewers can evaluate the dead-code cleanup independently.

plans/miles-port-unified-plan.md:4338 originally described miles_engine_index and miles_worker_url as the data source for _is_scheduler_preempt(output). That was the plan's original design: _is_scheduler_preempt was expected to read those two fields from router-injected response metadata.

The implementation later converged to a simpler contract. In multi_turn.py, _is_scheduler_preempt(output) only reads meta_info["miles_admission_disabled"]. It does not read miles_engine_index or miles_worker_url.

That means the plan was over-designed for the final implementation. The reader side was simplified, but the router writer side was never cleaned up, leaving one-sided dead metadata fields.

A repo-wide search confirms this:

• meta_info["miles_engine_index"] has no consumer.
• meta_info["miles_worker_url"] has no consumer.
• worker_engine_index_map only exists to support the unused miles_engine_index injection path.
• multi_turn only consumes miles_admission_disabled for redispatch classification.

Keeping the unused parser/storage/injection path makes the router admission code look more coupled than it is, and it diverges from the upstream-style minimal router surface without functional benefit.

Change

• Remove worker_engine_index_map from MilesRouter.
• Stop parsing engine_index from /add_worker query/body payloads.
• Stop injecting meta_info["miles_engine_index"] into /generate responses.
• Stop injecting unused meta_info["miles_worker_url"] into /generate responses.
• Remove worker_engine_index_map from the dev-only admission_state endpoint.
• Update tests and docstrings to reflect that the only required RLix router metadata field is miles_admission_disabled.

Validation

Passed syntax/whitespace checks:

python -m compileall miles/router/router.py miles/rollout/base_types.py tests/test_partial_sleep_wake.py
git diff --check

Passed stale-reference checks:

rg -n miles_engine_index miles tests -g *.py
rg -n miles_worker_url miles tests -g *.py
rg -n worker_engine_index_map miles tests -g *.py
# no matches

Passed affected router admission tests using a temporary local Ray import stub, because this Windows environment does not have the real Ray package installed:

python -m pytest tests/test_partial_sleep_wake.py::TestRouterAdmissionLifecycle -q
# 3 passed, 1 warning