[9/y] Fix nested optional codegen

[andrewchang-bird]

Stack:
📚 #248 ← [9/y] Fix nested optional codegen
📚 #247 [8/y] Fix wildcard arg matching for Obj-C param types
📚 #246 [7/y] Fix stubbing nil values in Obj-C mocks
📚 #254 [6/y] Update example projects
📚 #253 [5/y] Improve support for configuring SPM Xcode projects
📚 #252 [4/y] Show help message no mockable types are generated
📚 #251 [3/y] Fix unavailable generic protocol mock initializer
📚 #250 [2/y] Fix generator caching for multi-project setups
📚 #249 [1/y] Optimize dependency graph traversal
📚 #245 Replace SwiftPM with Swift Argument Parser

This fixes the implicit type coercion when bridging nested optional property setters to Any? for dynamic mocking.

Before

// warning: Expression implicitly coerced from 'String??' to 'Any?'
if let mkbImpl = mkbImpl as? (Any?) -> Any? { return Mockingbird.dynamicCast(mkbImpl(newValue)) as Void }

After

if let mkbImpl = mkbImpl as? (Any?) -> Any? { return Mockingbird.dynamicCast(mkbImpl(newValue as Any?)) as Void }