[Chore] Extract duplicated endpoint URL building logic into a helper method

[gcatanese]

Context

The endpoint URL building logic is duplicated across every method in all generated service classes (e.g., AccountHoldersApi, DirectDebitMandatesApi, and others). Each method repeats the same three-line pattern:

endpoint = '/some/{id}/path'.gsub(/{.+?}/, '%s')
endpoint = endpoint.gsub(%r{^/}, '')
endpoint = format(endpoint, id)

This was noted in a review comment on PR #350.

Proposed Change

Extract the repeated endpoint building logic into a private helper method in the base Service class or in the generated classes themselves. The Mustache templates in /templates should be updated to emit a call to this helper instead of inlining the three-step pattern:

def build_endpoint(path, *params)
  endpoint = path.gsub(/{.+?}/, '%s').delete_prefix('/')
  format(endpoint, *params)
end

Generated methods would then simplify to:

def get_tax_form_summary(id, headers: {}, query_params: {})
  endpoint = build_endpoint('/accountHolders/{id}/taxFormSummary', id)
  endpoint += create_query_string(query_params)
  action = { method: 'get', url: endpoint }
  @client.call_adyen_api(@service, action, {}, headers, @version)
end

Benefit

Reduces boilerplate in every generated service method, makes the templates easier to maintain, and lowers the risk of copy-paste mistakes when the URL construction logic needs to change in the future.