Filter nested generic definitions by all generic constraints

[VPDPersonal]

Summary

• Fix BuildParamPage over-broadening the nested open-generic list for a type parameter with multiple base constraints (where T : Base, IFoo).
• Instead of collapsing the constraints to a single type (or object) before fetching nested definitions, pass the full baseTypes to GetAssignableGenericDefinitions so the candidate defs are narrowed by every constraint up front — no more dead-end picks that end in an inline "Cannot construct…" error.

Notes for review

• GetAssignableGenericDefinitions(fieldType, params Type[] narrowTypes) checks closeability against fieldType and each narrowing type; GetConstraintBaseTypes never returns an empty array (it falls back to { typeof(object) }), so baseTypes[0] is safe and object entries are ignored by the narrowing filter.
• constraintType is left untouched — it still backs the PickerPage.ConstraintType used downstream.

Linked issues

Refs #49 - addresses review finding #49 (comment)