[Bug] blockHashFromNumber returns null after 10s when indexing permits are exhausted (0.43.0+)

Bug report

Since #6491 (0.43.0), Chain::block_pointer_from_number checks chain_store.block_ptrs_by_numbers before calling the RPC. If the block isn't in recent_blocks_cache, that lookup goes to blocks_from_store_by_numbers, which calls pool.get_permitted(). So it waits for the indexing semaphore (pool_size + GRAPH_EXTRA_QUERY_PERMITS), with no timeout.

The index-node resolver wraps the lookup in BLOCK_HASH_FROM_NUMBER_TIMEOUT (10s). When indexing holds every permit, blockHashFromNumber returns null and the RPC is never tried. 0.41.1 went straight to the RPC.

indexer-agent reports this as Failed to query graph node for blockHashFromNumber: no data returned (IE070), which blocks epoch resolution and POI closes. Epoch start blocks are hours old, so they always take the store path.

Repro: v0.45.0 against 0.41.1, pool_size = 2, mainnet and gnosis ingestors. To exhaust the permits I held LOCK TABLE public.ethereum_networks IN EXCLUSIVE MODE. Both ingestors then wait in the chain head UPDATE while holding their permit, which I confirmed in pg_stat_activity. It's a stand-in for write load.

store_semaphore_wait_ms doesn't show it. The wait is recorded only once the permit is acquired, and this lookup is cancelled before that (the metric read 52 during the null).

GRAPH_EXTRA_QUERY_PERMITS=4 does return the hash, in 5.11s, because the lookup times out on the pool connection and falls through to the RPC. That timeout calls state_tracker.mark_unavailable though, so it isn't a good workaround.

Suggested fix: drop the cache check from block_pointer_from_number, as #6537 did for is_on_main_chain, because the block cache isn't finality-aware here either. The callers are the index-node resolver, the registrar on deploy/graft and the public POI fallback, none of them hot. Still present on master (6838f4e3c).

Can open a PR with that if the direction sounds right?

Relevant log output

WARN Failed to fetch block hash from number, error: deadline has elapsed, block_number: 25961537, chain: ethereum, network: mainnet, component: IndexNodeServer > IndexNodeResolver
INFO Query timing (GraphQL), block: 0, query_time_ms: 10002, variables: null, query: { version { version } blockHashFromNumber(network: "mainnet", blockNumber: 25961537) } , subgraph_id: indexnode, component: IndexNodeServer

IPFS hash

No response

Subgraph name or link to explorer

No response

Some information to help us out

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OS information

macOS

Chain::block_pointer_from_number から開始し、chain_store.block_ptrs_by_numbers をたどって blocks_from_store_by_numbers まで進み、その後インデックスノードリゾルバの呼び出し元を調べます。インデックス作成用の permit が使い果たされた状態で再現し、blockHashFromNumber が RPC に到達して既存のタイムアウト内にハッシュを返すこと、また状態を利用不可としてマークしないことを確認します。関連するテストが存在する場合は、回帰テストのカバレッジを追加します。