Fix resolved loans blocking preparation count updates

[acwhite211]

Fixes #7625

The preparation count validator was counting historical resolved loans as if they were still active. This change updates the front-end check to only block decreases when there is unresolved quantity still on loan.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list

Testing instructions

• Create a preparation with count 3.
• Add that preparation to a loan with quantity 3.
• Return and resolve all 3 items on the loan.
• Open the preparation again.
• Change the preparation count from 3 to 1.
• Save the record.
• Confirm the save succeeds and the "The preparation is used in a loan." error does not appear.
• Reopen the same preparation and confirm the new count is still 1.

[emenslin]

• Confirm the save succeeds and the "The preparation is used in a loan." error does not appear.
• Reopen the same preparation and confirm the new count is still 1.

Looks good!

[bhumikaguptaa]

• Confirm the save succeeds and the "The preparation is used in a loan." error does not appear.
• Reopen the same preparation and confirm the new count is still 1.

--
Works as expected.