Warn and drop deprecated inputs instead of crashing

[hua7450]

Summary

When a partner posts a household payload containing a removed model variable, the simulation crashes with VariableNotFoundError → HTTP 500 and the partner sees no output at all — not just for the affected variable, but for everything they asked for. PR #1493 surfaced this with medical_out_of_pocket_expenses (deleted in policyengine-us 1.673.0); other deprecations will likely break the same way.

This PR adds a deprecation registry and a drop_deprecated_inputs helper that strips removed variables from the inbound payload before validation, surfacing a structured warning to the response next to the existing period warnings. The partner gets a 200 with full output for every non-dependent variable, plus an actionable migration hint.

It also handles deprecated variables used as scan axes. If axes[].name references a removed variable, that axis is dropped with a location-specific warning before the simulation builder expands axes, avoiding the same 500 path.

Behavior

Scenario	Before	After
Partner posts medical_out_of_pocket_expenses	HTTP 500, no output	HTTP 200, full output, warning in response body
Partner scans over axes[].name = medical_out_of_pocket_expenses	HTTP 500 during axis expansion	HTTP 200, deprecated axis dropped, warning in response body
Outputs that ignore that input (CTC, EITC, base SNAP eligibility, premium calculations, etc.)	Lost (whole request 500s)	Computed correctly
Outputs that depended on it (itemized medical deduction, SNAP excess medical deduction, HUD medical-expense deduction, Medicaid medically-needy spend-down, PA CCW deduction)	Lost (whole request 500s)	Fall back to default (often 0)

Tradeoff

This is a warn-and-drop, not a value-preserving shim. The partner's value is silently discarded; outputs that need that input fall back to defaults. For partners who passed 0 (e.g. MyFriendBen pre-#1493), this is a no-op semantically. For partners who passed a non-zero value with premium spend folded in, they'll need to migrate the premium portion to health_insurance_premiums to recover those outputs — the warning hint says exactly that.

Files

• policyengine_household_api/utils/deprecated_inputs.py — new registry + helper for entity inputs and scan axes
• policyengine_household_api/endpoints/household.py — wire the helper into /calculate before validation; merge warnings into the existing response field
• tests/unit/utils/test_deprecated_inputs.py — unit coverage for drop+warn, no-deprecated passthrough, multi-person, axes safety, deprecated axis dropping, members-list safety, and message format
• tests/unit/endpoints/test_household.py — end-to-end coverage confirming requests with medical_out_of_pocket_expenses as an input or axis return 200 + warning + correct unaffected outputs
• changelog.d/warn-and-drop-deprecated-inputs.added.md — changelog fragment

Future deprecations

Each new entry in DEPRECATED_VARIABLES ships with a one-line migration hint. The registry is the single place to add future removals (e.g. when other umbrella inputs are decomposed) so we don't have to recur through this design.

Test plan

• Unit tests cover drop + warning shape, multi-person, list-valued slots, and deprecated scan axes
• Endpoint tests confirm HTTP 200 + warnings + non-medical outputs unaffected
• uv run pytest tests/unit/utils/test_deprecated_inputs.py tests/unit/endpoints/test_household.py
• uv run ruff check policyengine_household_api/utils/deprecated_inputs.py tests/unit/utils/test_deprecated_inputs.py tests/unit/endpoints/test_household.py
• CI passes

🤖 Generated with Claude Code

[anth-volk]

This is a critical fix, so let's merge, but I want to flag: drop_deprecated_inputs directly mutates the household instead of deep-copying, opening us up to a class of unexpected/unclear bugs down the road. Recommend making this non-mutative via deep-copy in a future PR.