Upgrade a2a-go to v2

[pavelanni]

Summary

Upgrade from github.com/a2aproject/a2a-go v0.3.x to github.com/a2aproject/a2a-go/v2.

Motivation

a2a-go v2 has been released with breaking API changes (e.g., AdditionalInterfaces → SupportedInterfaces, updated type signatures). Staying on v1 means we miss upstream improvements and protocol updates.

Scope

• Update all imports from github.com/a2aproject/a2a-go/a2a to github.com/a2aproject/a2a-go/v2/a2a
• Update internal/bridge/ (client, executor, agentcard, message, delegation)
• Update internal/cmd/serve.go and internal/cmd/chat.go
• Adapt to any type/method signature changes
• Run full test suite

Related

• Interactive CLI chat client with Bubble Tea TUI #11 — CLI chat client (uses a2a types for Agent Card)