fix: add missing hotkey guard in handle_pat_check to prevent ValueError DoS

[Tet-9]

Summary

handle_pat_check called validator.metagraph.hotkeys.index(hotkey)
without first verifying the hotkey is still registered. When a
metagraph refresh deregisters a hotkey between the blacklist pass and
handler execution — a real TOCTOU window at epoch boundaries —
list.index() raises an uncaught ValueError that crashes the axon
coroutine. The validator stops processing all PatCheckSynapse
requests until manually restarted.

Root Cause

The axon pipeline runs blacklist → priority → handler as separate
async calls with no atomic lock over the metagraph. A hotkey that
passes blacklist_pat_check can be deregistered before
handle_pat_check executes, hitting the unguarded .index() call.

Every sibling function in pat_handler.py already guards this:

Function	Guard
handle_pat_broadcast	✅
blacklist_pat_broadcast	✅
priority_pat_broadcast	✅
blacklist_pat_check	✅
priority_pat_check	✅
handle_pat_check	❌ missing

Fix

Apply the same guard already used in handle_pat_broadcast — check
hotkey not in validator.metagraph.hotkeys before calling .index(),
and return a rejected synapse with rejection_reason='Hotkey not registered on subnet' on the missing-hotkey path.

Testing

1475 tests passing, no regressions.

Fixes #1297