test: resolve test_helpers basename collision via rename

Revert the three __init__.py files added in Phase 3 and rename
tests/api/test_helpers.py -> tests/api/test_api_helpers.py to
disambiguate it from tests/tools/transaction/test_helpers.py under
pytest's prepend import mode. This keeps the test tree flat and
consistent (no partial-package asymmetry) instead of relying on a
subset of __init__.py files. Update the AGENTS.md tree accordingly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: akolotov

AGENTS.md

@@ -108,7 +108,7 @@ mcp-server/
 │   │       ├── test_get_transaction_info_real.py  # Integration tests for get_transaction_info
 │   │       └── test_get_transactions_by_address_real.py  # Integration tests for get_transactions_by_address
 │   ├── api/                      # Unit tests for the REST API
-│   │   ├── test_helpers.py       # Unit tests for REST error-handling helpers (e.g. handle_rest_errors)
+│   │   ├── test_api_helpers.py   # Unit tests for REST error-handling helpers (e.g. handle_rest_errors)
 │   │   ├── test_resource_routes.py  # Unit tests for resource discovery routes (/v1/resources)
 │   │   ├── test_routes.py        # Unit tests for API route definitions
 │   │   └── test_skill_resource_routes.py  # Unit tests for the bundled skill HTTP mirror