server: reject unreachable store addresses in PutStore

## Bug Report

### What did you do?

Register a store through `PutStore` with a store address that is syntactically valid but not reachable by TCP dial.

### What did you expect to see?

PD should reject the store registration with `ErrorType_INVALID_VALUE` when the configured store address cannot be dialed.

### What did you see instead?

PD accepted the store registration and persisted the unreachable address. This can leave PD with an incorrect TiKV/TiFlash store address.

### What version of PD are you using (`pd-server -V`)?

master

Start at the server-side PutStore handler and trace the existing store-address validation and registration path. Reproduce the case with a syntactically valid address that cannot be reached by TCP, then verify that registration is rejected with ErrorType_INVALID_VALUE and the unreachable address is not persisted.

Written by the indexing model from the issue text.