Add Code Action Add Missing Method Implementations

[amanthatdoescares]

Add Objective-C Method Implementation CodeActions

Problem

SourceKit-LSP provides code actions for Swift incomplete implementations but lacks this for Objective-C files. Xcode offers this functionality natively, and SourceKit-LSP should match this developer experience.

Current State

• Basic AddMethodImplementation infrastructure exists in Sources/SwiftLanguageService/CodeActions/AddMethodImplementation.swift
• Connected to main service but returns empty arrays (incomplete implementation)
• No Objective-C specific parsing or method stub generation

Fixes: #2186

[ahoppen]

This sort of string-based scraping for error messages seems very brittle. Both clang and swift can already provide Fix-Its for diagnostics and offer them as code actions to the user. We should use those instead of synthesizing them in SourceKit-LSP.

The real task is figure out why the Fix-It might not have shown up in #2186 and, if that root cause has been identified, fix it.