refactor(miles): share router URL acquisition logic

[TianyeGGBond]

Context

MilesRouter._use_url() is the sync selector used by direct tests, while _use_url_async() is the production proxy selector. Both paths had the same two-line acquisition logic:

url = min(candidates, key=lambda u: self.worker_request_counts.get(u, 0))
self.worker_request_counts[url] += 1

That duplication is risky because the tested sync path and production async path can drift. The async path also has an important locking invariant: the in-flight count increment must remain inside _workers_changed's condition lock.

Change

• Add _pick_least_loaded(candidates) to own the shared "select least-loaded URL + increment in-flight count" operation.
• Use it from both _use_url() and _use_url_async().
• Keep _use_url_async() calling the helper inside the existing condition lock, preserving the concurrency invariant.
• Add a fast router test for _use_url_async() so the production selector path has direct coverage for the same least-load behavior already tested on _use_url().

Risk

Low. This is a mechanical refactor of identical code. It should not change Ray/runtime behavior because:

• _candidate_set() is unchanged.
• _use_url() still raises on empty candidates.
• _use_url_async() still waits for a non-empty candidate set.
• _use_url_async() still increments the request count while holding _workers_changed's lock.

The main risk would be accidentally moving the async increment outside the lock; this PR does not do that.

Validation

Passed syntax/whitespace checks:

python -m compileall miles/router/router.py tests/fast/router/test_router.py
git diff --check origin/zhenyu/m11-mvp-test..HEAD

Passed a focused local selector check with a temporary Ray import stub:

assert router._use_url() == "http://w2:8000"
assert router.worker_request_counts["http://w2:8000"] == 3
assert asyncio.run(router._use_url_async()) == "http://w2:8000"
assert router.worker_request_counts["http://w2:8000"] == 3

Local pytest limitation:

python -m pytest tests/fast/router/test_router.py::TestLoadBalancing -q

This Windows environment cannot collect the fast router test module without the full project dependencies installed. Collection is blocked by missing packages (ray first, then sglang after a temporary Ray stub). The new test is still included for CI/full dev environments where the fast router suite dependencies are present.