## Context

While testing the `GET /api/v1/customers` endpoint, I noticed that [the documentation](https://github.com/getlago/lago-openapi/blob/11d325bf076e6381c00674ca3b03de3c4eca845c/src/resources/customers.yaml#L36-L58) mentions `account_type` and `billing_entity_codes` parameters as arrays, but document them without the trailing `[]` brackets. This was different from other endpoints such as [the subscription one](https://github.com/getlago/lago-openapi/blob/11d325bf076e6381c00674ca3b03de3c4eca845c/src/resources/subscriptions.yaml#L54-L68).

So I tested the endpoint with `?billing_entity_codes=hooli&billing_entity_codes=gan` and `?billing_entity_codes=hooli,gan` and it returned a 500 error.

Using `?billing_entity_codes[]=hooli&billing_entity_codes[]=gan` did work though. So it turned out to be a documentation issue, along with a missing sanitization on the parameter. The fix for the doc can be found here: getlago/lago-openapi#388.

## Description

I fixed the sanitization on the `billing_entity_codes` parameter and then added a test to ensure it does not return a 500 error without the trailing `[]` brackets.

When doing so, I noticed that passing multiple identical billing entity codes did return a 404 error instead of a 200 one. So I fixed this as well along with a test.

Below are the outputs of the tests without the fixes:

```
1) Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with invalid billing entity codes ignores the parameter
Failure/Error: return not_found_error(resource: "billing_entity") if params[:billing_entity_codes].present? && billing_entities.count != params[:billing_entity_codes].count

ArgumentError:
  wrong number of arguments (given 0, expected 1+)

2) Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with two identical billing entity codes returns customers for the specified billing entity
Got 1 failure and 1 other error:

2.1) Failure/Error: expect(response).to have_http_status(:ok)
       expected the response to have status code :ok (200) but it was :not_found (404)
     # ./spec/requests/api/v1/customers_controller_spec.rb:434:in 'block (5 levels) in <top (required)>'
     # ...
     # /usr/local/bundle/gems/webmock-3.23.1/lib/webmock/rspec.rb:39:in 'block (2 levels) in <main>'

2.2) Failure/Error: expect(json[:customers].count).to eq(1)

     NoMethodError:
       undefined method 'count' for nil
     # ./spec/requests/api/v1/customers_controller_spec.rb:435:in 'block (5 levels) in <top (required)>'
     # ....
     # /usr/local/bundle/gems/webmock-3.23.1/lib/webmock/rspec.rb:39:in 'block (2 levels) in <main>'

Finished in 2.7 seconds (files took 2.47 seconds to load)
51 examples, 2 failures

Failed examples:

rspec ./spec/requests/api/v1/customers_controller_spec.rb:419 # Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with invalid billing entity codes ignores the parameter
rspec ./spec/requests/api/v1/customers_controller_spec.rb:431 # Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with two identical billing entity codes returns customers for the specified billing entity
```

## Context

While testing the `GET /api/v1/customers` endpoint, I noticed that [the documentation](https://github.com/getlago/lago-openapi/blob/11d325bf076e6381c00674ca3b03de3c4eca845c/src/resources/customers.yaml#L36-L58) mentions `account_type` and `billing_entity_codes` parameters as arrays, but document them without the trailing `[]` brackets. This was different from other endpoints such as [the subscription one](https://github.com/getlago/lago-openapi/blob/11d325bf076e6381c00674ca3b03de3c4eca845c/src/resources/subscriptions.yaml#L54-L68).

So I tested the endpoint with `?billing_entity_codes=hooli&billing_entity_codes=gan` and `?billing_entity_codes=hooli,gan` and it returned a 500 error.

Using `?billing_entity_codes[]=hooli&billing_entity_codes[]=gan` did work though. So it turned out to be a documentation issue, along with a missing sanitization on the parameter. The fix for the doc can be found here: getlago/lago-openapi#388.

## Description

I fixed the sanitization on the `billing_entity_codes` parameter and then added a test to ensure it does not return a 500 error without the trailing `[]` brackets.

When doing so, I noticed that passing multiple identical billing entity codes did return a 404 error instead of a 200 one. So I fixed this as well along with a test.

Below are the outputs of the tests without the fixes:

```
1) Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with invalid billing entity codes ignores the parameter
Failure/Error: return not_found_error(resource: "billing_entity") if params[:billing_entity_codes].present? && billing_entities.count != params[:billing_entity_codes].count

ArgumentError:
  wrong number of arguments (given 0, expected 1+)

2) Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with two identical billing entity codes returns customers for the specified billing entity
Got 1 failure and 1 other error:

2.1) Failure/Error: expect(response).to have_http_status(:ok)
       expected the response to have status code :ok (200) but it was :not_found (404)
     # ./spec/requests/api/v1/customers_controller_spec.rb:434:in 'block (5 levels) in <top (required)>'
     # ...
     # /usr/local/bundle/gems/webmock-3.23.1/lib/webmock/rspec.rb:39:in 'block (2 levels) in <main>'

2.2) Failure/Error: expect(json[:customers].count).to eq(1)

     NoMethodError:
       undefined method 'count' for nil
     # ./spec/requests/api/v1/customers_controller_spec.rb:435:in 'block (5 levels) in <top (required)>'
     # ....
     # /usr/local/bundle/gems/webmock-3.23.1/lib/webmock/rspec.rb:39:in 'block (2 levels) in <main>'

Finished in 2.7 seconds (files took 2.47 seconds to load)
51 examples, 2 failures

Failed examples:

rspec ./spec/requests/api/v1/customers_controller_spec.rb:419 # Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with invalid billing entity codes ignores the parameter
rspec ./spec/requests/api/v1/customers_controller_spec.rb:431 # Api::V1::CustomersController GET /api/v1/customers when filtering by billing_entity_code with two identical billing entity codes returns customers for the specified billing entity
```