fix: raise ProviderUnreachableError from get_balance so outages don't read as zero balance

[edwin-rivera-dev]

Summary

BitcoinProvider.blockstream_get_balance and SubtensorProvider.get_balance returned 0 on any backend failure. The validator axon at handle_swap_reserve checks balance < from_amount — so during a Blockstream or subtensor RPC outage, every legitimate user is rejected with "Insufficient source balance" regardless of their actual funds.

Change

• chain_providers/base.py — abstract get_balance docstring: documents the ProviderUnreachableError contract.
• chain_providers/bitcoin.py — blockstream_get_balance raises ProviderUnreachableError on ConnectionError/Timeout/HTTPError, matching blockstream_verify_transaction.
• chain_providers/subtensor.py — get_balance raises ProviderUnreachableError on any subtensor failure (non-existent addresses return zero cleanly from RPC, so exceptions mean backend unreachable).
• validator/axon_handlers.py — handle_swap_reserve catches ProviderUnreachableError, logs a warning, and skips the balance gate.

Closes #147

[edwin-rivera-dev]

🐛 Fixed
Hi @anderdc , How are you? Could you review this PR please?
Thanks.