perf: parallelize sequential awaits with asyncio in async_subtensor

[altis0725]

Fixes #3129

Summary

Apply asyncio.gather to three methods to run sequential RPC calls concurrently:

• get_all_dynamic_info: runtime_call and get_subnet_prices in parallel
• blocks_since_last_update: get_block_number and get_hyperparameter in parallel, with pre-resolved block_hash for chain state consistency
• blocks_until_next_epoch: dynamic gather for get_block_number and tempo when both need fetching

Each parallelized pair saves ~1 RPC round-trip (50-200ms depending on node latency).

Test plan

• Updated test_blocks_since_last_update_success and test_blocks_since_last_update_no_last_update for new calling convention
• CI passes

[basfroman]

Could you pls explain the real benefit of this pr?

[altis0725]

Hi @basfroman, thanks for the feedback. I've rebased the branch to a single clean commit.

This PR implements the two items from #3129:

1. asyncio.gather applied to 3 methods — each saves ~1 RPC round-trip by running sequential substrate calls concurrently
2. determine_block_hash improvement — pre-resolve block_hash in blocks_since_last_update before parallel calls to ensure both reference the same chain state

The previous version had too many incremental commits that obscured the intent — apologies for the noise.