TASK-189246 lock gc before traversing openupval

[balupillai]

Note

Medium Risk
Touches core GC/stop-the-world synchronization and upvalue lifecycle; small diff but incorrect blocking/unblocking or recursion balancing could introduce deadlocks or GC stalls under load.

Overview
Prevents a concurrency race between global GC tracing and mutation of a thread’s open-upvalue list by wrapping luaF_close and open-path luaF_freeupval unlinking with luaC_blockcollector/luaC_unblockcollector.

Adds per-thread recursion accounting to the global-trace mutator stop/start path (try_block_mutators/unblock_mutators) so nested collector-blocking operations remain balanced.

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