test(code-interpreter/python): cover async endpoint headers

[Gujiassh]

Summary

• add async Python code-interpreter regression coverage proving SandboxEndpoint.headers reaches both the generated OpenAPI client and the SSE execution path
• lock endpoint-header precedence over connection-level headers so future parity changes do not regress the current async behavior
• keep this follow-up test-only and scoped to the Python async package while the sync parity fix is handled separately in fix(code-interpreter/python): apply endpoint headers in sync adapter #504

Validation

• cd sdks/code-interpreter/python && uv run pytest tests/test_adapter_eager_init.py tests/test_code_service_adapter_streaming.py -q
• cd sdks/code-interpreter/python && uv run ruff check tests/test_adapter_eager_init.py tests/test_code_service_adapter_streaming.py