protocol: add InitializeResult extension metadata

[roblourens]

Summary

• add optional InitializeResult._meta for namespaced, implementation-specific extension capabilities
• document unknown-key handling and when capabilities should graduate to typed protocol fields
• regenerate schemas and all client mirrors
• add a shared conformance fixture proving nested metadata round-trips across clients

This supports the VS Code-owned chat-targeted state-file capability needed by microsoft/vscode#332820.

Validation

• npm run generate
• npm run test (440 passed; 100% reducer coverage)
• TypeScript round-trip corpus (3 passed)
• Go round-trip corpus
• Rust round-trip corpus (1 passed)
• Swift round-trip corpus (1 passed)
• Kotlin round-trip corpus (all 44 fixtures passed)
• .NET test project (535 passed)

(Written by Copilot)