fix(balance): clear cached P2C ready index after a discovery removal

[ameyypawar]

Summary

Fixes #856.

p2c::Balance caches the P2C-selected endpoint as a numeric ready_index. When a subsequent poll_ready processes a Change::Remove, update_pending_from_discover calls ReadyCache::evict, which removes from the ready set with swap_remove_full. That can move a different endpoint into the slot the cached ready_index points at. poll_ready then revalidates the stale index with check_ready_index (which succeeds for whatever now occupies that slot) and call dispatches the request there — silently bypassing the P2C choice during endpoint churn.

This matches the warning already in ready_cache's own docs:

calls to ReadyCache::poll_pending and ReadyCache::evict may perturb the order of the ready set, so any cached indexes should be discarded after such a call.

Fix

Clear ready_index whenever a removal evicts a service, so the next poll_ready re-runs P2C over the current ready set:

Some(Change::Remove(key)) => {
    trace!("remove");
    self.services.evict(&key);
    // `evict` removes from the ready set with `swap_remove`, which can move a
    // different endpoint into the slot a previously-selected `ready_index`
    // points at. Discard the cached selection so `poll_ready` re-runs P2C.
    self.ready_index = None;
}

I also corrected the poll_ready comment that claimed "These updates cannot disturb the order of existing ready services" — they can, via swap_remove.

Why this minimal fix is sufficient

The only operation that reorders the existing ready set between caching ready_index and reusing it is evict's swap_remove_full:

• ReadyCache::poll_pending only inserts into the ready IndexMap (appends new keys / updates existing in place — existing indices stay put).
• Change::Insert pushes to the pending set, never the ready set.
• check_ready_index's not-ready/error arms and call_ready_index also swap_remove, but only after ready_index has been .take()n within the same poll_ready loop, each immediately followed by a fresh P2C selection — so no stale index survives across calls.

The issue notes a more robust alternative (cache the key instead of the index); that's a larger change and isn't required to close this bug, but happy to go that route if you'd prefer it.

Tests

Not included in this commit. A deterministic regression test needs a dynamic Discover (to emit Change::Remove between poll_ready calls) plus a seeded Rng, which the current p2c tests don't have scaffolding for. Glad to follow up with that test (building the small test Discover + mock Rng) if you'd like it in this PR.