fix: filters for only active policies for spend_limit check

[brobro10000]

Updates the spend_limit constraint check to only check on policies that are active and to determine the sum of the spend_limit's on only active policies.
Also updates how we sum sibling policies in the total_spend_limit_for_all_policies_associated_to_subsidy function by using the ORM to determine the sum of siblings and adding together the spend_limit in memory.

Description:
Add a description of your changes here.

Jira:
ENT-XXXX

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[brobro10000]

Nested this logic in the conditional to check to see if the policy we are modifying is not active, we do not want to include it in the policy balances. This is in parity with the changes to the filtering that only looks for active sibling policies.

[pwnage101]

Non-blocking idea:

FWIW, there's probably a way in pure ORM to this entire function all in one line and one SQL query without having to exclude self.uuid or loop over the results, e.g.:

return SubsidyAccessPolicy.objects.filter(
    enterprise_customer_uuid=self.enterprise_customer_uuid,
    subsidy_uuid=self.subsidy_uuid,
    active=True,
).aggregate(Sum("spend_limit", default=0))["spend_limit__sum"]

Theoretically when this encounters a NULL spend_limit, The default=0 takes over. Just an Idea that could make this function less complex and easier to understand/debug.

[pwnage101]

Based on the standup discussion, it sounds like you want your clean function to say this instead:

if self.is_spend_limit_updated and not self.active:

[brobro10000]

Modified the conditional to avoid the path of a user disabling the policy. Setting an out of bounds spend_limit, then re-enabling the policy.
Before conditional added:
https://github.com/openedx/enterprise-access/assets/82611798/16a80f8d-de14-43ac-8ba3-efd71f9e5751

Screen.Recording.2024-06-24.at.2.10.02.PM.mov