design(mcp): build_po_modify_ui — diff-decorated PO modify card (#721 child)

[dougborg]

Child of #721 (modify-card redesign umbrella). Applies the #551 framework (entity-view + state machine + status badges + shared tier-1/4 helpers) to purchase-order modify / delete / correct.

Scope

Single new entrypoint build_po_modify_ui(response, *, confirm_request, confirm_tool) in prefab_ui.py.

Covers callers:

• modify_purchase_order (foundation/purchase_orders.py)
• delete_purchase_order (foundation/purchase_orders.py)
• correct_purchase_order (foundation/corrections.py)

All three currently route through _modification.to_tool_result → build_modification_preview_ui / build_modification_result_ui. The dispatch in _modification.to_tool_result adds an entity_type == \"purchase_order\" branch that calls build_po_modify_ui.

Content

Same entity view shape as build_po_create_ui from #551 — reuses _PO_FIELD_SPEC — decorated with:

• Diff display per changed field: before → after rendering driven by the changes parameter to _render_entity_view. The FieldChangeView payload comes from a converter that flattens ModificationResponse.changes per action.
• Per-field status pills in applied state: each changed field shows APPLIED / APPLIED (verified) / FAILED per the corresponding ActionResult.succeeded + verified flags.
• Operation grouping is implicit: header-field changes appear under the Header group; row additions/deletions appear under the Line Items group with added/removed kind; additional-cost rows under their own group.

Title verb varies by operation discriminator:

• modify → "Purchase Order Modify"
• delete → "Purchase Order Delete"
• correct → "Purchase Order Correct"

Confirm label + operation_label follow.

Tasks

• Implement build_po_modify_ui using shared helpers from design(mcp): redesign build_order_preview_ui per #537 four-tier framework (PO / SO / MO) #551.
• Implement _action_results_to_field_changes (or scope local to PO if cleaner).
• Implement _action_results_to_field_statuses (parallel — drives the applied-state per-field pills).
• Add entity_type == \"purchase_order\" branch in _modification.to_tool_result.
• Tests: TestBuildPOModifyUI with preview / applied / per-action-status / diff-decoration coverage. Field-spec parity test — every field in _PO_FIELD_SPEC that's non-None in the fixture appears in the rendered card.

Acceptance

• build_po_modify_ui renders the full PO entity view with diff decoration for changing fields and per-field status in applied state.
• No "Update Header" / "Add Additional Cost" / "N field(s) changed" terminology in the rendered card.
• Each of modify_purchase_order / delete_purchase_order / correct_purchase_order dispatches to build_po_modify_ui via _modification.to_tool_result.
• Existing tests for these three tools pass.