Speed up TestCreditLines by avoiding CouchDB user operations

[dannyroberts]

Product Description

N/A — test-only change.

Technical Summary

TestCreditLines was taking 200+ seconds in CI due to CouchDB operations:
creating CommCareUser documents, querying CouchDB views to count them via
calculate_users_in_all_domains, and deleting them in teardown.

The invoicing code only reads DomainUserHistory SQL records, so we can
create those directly instead of round-tripping through CouchDB. This follows
the same pattern already used by TestSubscriptionChangeTransfersSubscriptionLevelCredit
in the same file.

Also removes _generate_users_fee_to_credit_against which was dead code
(defined but never called).

Feature Flag

N/A

Safety Assurance

Safety story

Test-only change. The refactored tests exercise the same invoicing and credit
line logic — the only difference is how DomainUserHistory records are
created (directly via SQL instead of via CouchDB user creation + counting).

Automated test coverage

The 5 test methods in TestCreditLines themselves verify the behavior is
unchanged.

QA Plan

CI passing is sufficient.

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[dannyroberts]

(This comment was generated by Claude, but I've reviewed it and stand behind it—this is my current best understanding.)

Investigation results

The 200+ second setup time attributed to TestCreditLines in CI is not caused by the test itself. It's the one-time DeferredDatabaseContext.setup_databases() cost (CouchDB, Elasticsearch, BlobDB, SQL database initialization) that gets charged to whichever TestCase runs first in each shard.

Evidence: all three shards show the same ~200s setup on their first DB test, regardless of which test it is:

Shard	First DB test	Setup time
bf	TestCreditLines	183.95s
6a	TestUpgradeSoftwarePlanToLatestVersion	204.70s
05	TestKeepSoftwarePlanConsistentManagementCommand	211.94s

The changes in this PR (replacing CouchDB CommCareUser creation with direct DomainUserHistory SQL records) are a valid ~20s improvement to the per-test overhead, but they don't address the headline number since that's infrastructure setup.

Closing since the original premise (TestCreditLines setup being unusually slow) turned out to be a misattribution.