server, client: expose store scheduling state in GetStore

[LykxSassinator]

What problem does this PR solve?

Issue Number: Ref tikv/client-go#2012

PD owns the slow-store eviction and recovery-time policy, but the existing
GetStore path only exposes metapb.Store and drops fields carried by the
full GetStoreResponse. Consequently, downstream clients cannot distinguish a
locally recovered Store from one that PD still keeps evicted as slow.

This PR exposes PD's scheduling decision through GetStoreResponse, enabling
client-go proposal 3 to keep non-leader replica reads excluded until PD
explicitly re-admits the Store.

Depends on pingcap/kvproto#1503.

What is changed and how does it work?

• PD leader's GetStore response now populates
  StoreSchedulingState.evicted_as_slow_store from
  core.StoreInfo.EvictedAsSlowStore().
• Adds optional pd.RPCClientExt.GetStoreResponse, which returns the complete
  protobuf response while keeping the existing RPCClient and
  Client.GetStore APIs source-compatible.
• Keeps GetStore behavior unchanged by implementing it on top of
  GetStoreResponse and retaining existing response validation.
• Router responses deliberately leave SchedulingState unset: Router's
  metadata cache does not retain PD leader's dynamic scheduling decision, and
  reporting a default false value would incorrectly imply that the Store is
  eligible.
• Adds coverage for PD leader state exposure and Router's unknown semantics.

The kvproto dependency is temporarily replaced with the branch for PR #1503.
The replacement should be removed and dependencies tidied after that PR merges.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP APIs changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

PD client now exposes Store scheduling state in the full GetStore response so
clients can honor PD slow-store recovery decisions.

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