fix: Improve test coverage for /accounts/* endpoints

[eugypalu]

This PR improves integration test coverage for all /accounts/* endpoints by:

• Adding historical and latest fixture tests
• Introducing missing tests (including a full new suite for foreign-asset-balances)

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Coverage Matrix

Scenario	balance_info	asset_balances	pool_asset_balances	pool_asset_approvals	vesting_info	proxy_info	foreign_asset_balances
basic	✅	✅	✅	✅	✅	✅	✅ new
at_specific_block	✅	✅	✅	✅	✅	✅	✅ new
invalid_address	✅	✅	✅	✅	✅	✅	✅ new
hex_address	✅	✅ new	✅	✅ new	✅ new	✅	✅ new
response_structure	✅	✅	✅ new	✅ new	✅	✅ new	✅ new
use_rc_block	✅	✅	✅	✅	✅	✅	✅ new
use_rc_block_empty	✅	✅	✅ new	—	✅	—	✅ new
with_filter	—	✅	✅	—	—	—	✅ new
invalid_filter	—	—	—	—	—	—	✅ new
pallet_not_available	—	—	—	—	—	—	✅ new

closes #246