test: move policyId validation to account endpoint tests
Move test_tokens_expose_policy_id from test_token_registry.py to test_account_endpoints.py as it validates API contract (policyId always present for native assets) rather than token registry enrichment. The test was incorrectly passing when token registry was misconfigured because policyId is populated from blockchain data, not registry metadata. Changes: - Add TestNativeAssetMetadata class to test_account_endpoints.py - Remove TestPolicyIdMetadata class from test_token_registry.py - Enhanced validation to check all native assets, not just configured tokens - Added policyId format validation (56 hex chars)