Fix use of User#expires_at in SpecHelpers#stub_auth_for (#82)

Previously, if `User#expires_at` was defined, but it was set to `nil`,
the `expires_in` calculation was returning a large negative integer.
This was because `nil.to_i` is 0 and thus `user.expires_at.to_i -
Time.zone.now.to_i` was a value like `-1744820986` (for 2025-04-16
17:30) which meant the credentials were effectively immediately expired
some time ago.

This isn't a problem in e.g. `code-club-frontend`, because [the user
factory sets `User#expires_at` to a time a random number of seconds in
the future, i.e. `rand(60..300).seconds.from_now][1].

We've had to do [something similar in `experience-cs`][2], but I think a
better solution is to default to using the 3600 fallback value if
`User#expires_at` is `nil` just like we do if `User#expires_at` is not
defined.

[1]:
https://github.com/RaspberryPiFoundation/code-club-frontend/blob/f7e965798d910584fed0d1eb7867f32a899f9ce8/spec/factories/user.rb#L21
[2]:
https://github.com/RaspberryPiFoundation/experience-cs/blob/47d11bd60da7b7bd93968534542f84edca4054aa/spec/factories/user.rb#L5

Author: floehopper

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 ### Fixed
+- Fix use of `User#expires_at` in `SpecHelpers#stub_auth_for` (#82)
 
 ### Removed
 

lib/rpi_auth/spec_helpers.rb

@@ -6,7 +6,8 @@ module SpecHelpers
     # model has an `:user_id` method which returns the users ID, but this can
     # be changed by setting the `id_param` option.
     def stub_auth_for(user:, id_param: :user_id) # rubocop:disable Metrics/AbcSize
-      expires_in = user.respond_to?(:expires_at) ? user.expires_at.to_i - Time.zone.now.to_i : 3600
+      expires_at = user.respond_to?(:expires_at) && user.expires_at
+      expires_in = expires_at.present? ? expires_at.to_i - Time.zone.now.to_i : 3600
       token = user.respond_to?(:access_token) ? user.access_token : SecureRandom.hex(16)
       refresh_token = user.respond_to?(:refresh_token) ? user.refresh_token : SecureRandom.hex(16)