refactor: Replaces the hand-rolled NNS state-tree parsing for subnet types and canister ranges with Agent::fetch_subnet_by_id from ic-agent

[shilingwang]

#NODE-1911

Summary

Replaces the hand-rolled NNS state-tree parsing for subnet types and canister ranges with Agent::fetch_subnet_by_id from ic-agent.
Before: SubnetsInfoFetcher performed three distinct NNS round trips per cycle:

• Read NNS /subnet to get all subnet IDs and their types (hand-parsed from the hash tree)
• Read NNS /canister_ranges/ once per subnet (CBOR-decoded manually)

After: The fetcher still reads NNS /subnet to discover subnet IDs, then calls agent.fetch_subnet_by_id concurrently for each subnet, which retrieves both the type and canister ranges from each subnet's own state tree in a single round trip per subnet — with all certificate verification and tree parsing handled by ic-agent.

Changes

• fetch_subnets → fetch_subnet_ids: now only extracts subnet IDs from the NNS /subnet subtree; type extraction removed
• fetch_canister_ranges removed: replaced by agent.fetch_subnet_by_id calls inlined into fetch()
• map_subnet_type added: thin free function mapping ic-agent's SubnetType to the local enum, keeping the local type Copy and the public API decoupled from ic-agent internals
• From<&[u8]> for SubnetType removed: no longer needed since raw byte parsing is handled by ic-agent
• Test fixtures removed: testdata/subnet.bin, testdata/nns_canister_ranges.bin, and all related test infrastructure (TESTNET_ROOT_KEY, read_state_response, the extended ingress-expiry workaround). Tests that required real BLS-signed certificates are replaced by a pure in-memory subnet_type_lookup test that exercises SubnetsInfo::new → ArcSwapOption::store → load → subnet_type() directly.