Bug: Policy parameter changes (especially booleans) not saved to API - policy_json is empty

Update

Root cause identified: This was user error, not a code bug. The user changed the parameter value but didn't click "Add parameter" to apply it. See #604 for UX improvements to make this flow clearer.

Original Description

When creating a policy with parameter modifications (tested with boolean switches), the policy is created but the policy_json is empty, meaning no parameter changes are actually saved.

Steps to Reproduce

1. Start creating a new report or policy
2. Select a boolean parameter (e.g., gov.irs.deductions.itemized.charity.floor.applies - "AGI floor on charitable deduction")
3. Change the boolean value (e.g., from true to false)
4. Click "Add parameter" ← This step was missed
5. Review and submit the policy
6. The report shows "no impact" and "no policy parameters have changed"

Resolution

• The real fix is #604 (UX improvement to make Add parameter more obvious) and #605 (block creating policies with no changes)
• PR #603 still provides value as a code quality improvement (better immutability, unit tests) even though it doesn't fix the reported issue

Start by reading the updates and the linked work in issues #604 and #605; this issue identifies the missed “Add parameter” action as the cause and says those issues contain the UX and validation fixes. Review PR #603 only for its separate code-quality and unit-test changes; this issue is done when the reported behavior is handled by that follow-up work.