feat: replace custom ContextStore with A2A-native task history

[jezekra1]

Summary

• Replace the custom ContextStore abstraction with A2A's native TaskStore as the single source of truth for conversation history
• Implement on_list_tasks in the A2A proxy so the UI can fetch history via ListTasks JSON-RPC
• Rewire the UI to fetch conversation history from A2A tasks instead of the /contexts/{id}/history REST endpoint
• Remove ~1,200 lines of duplicate history storage code

This is a breaking change — the context_store parameter, context.store(), and /contexts/history endpoints are all removed.

Closes #116

Changes by layer

adk-server

• Implement on_list_tasks in ProxyRequestHandler (forwards to agent transport)
• Remove context history endpoints (POST/GET/DELETE /contexts/{id}/history)
• Remove title generation job and worker queue
• Remove history-related service methods, repository code, and schema types
• Keep context CRUD endpoints for sidebar/metadata

adk-py

• Delete ContextStore, InMemoryContextStore, PlatformContextStore
• Rewire RunContext to use TaskStore — load_history() now reads from task_store.get(task_id)
• Remove store(), store_sync(), delete_history_from_id() from RunContext
• Remove context_store parameter from create_app() and Server.serve()
• Remove ContextHistoryItem and history methods from platform SDK
• Update all agents, examples to remove context.store() calls

adk-ui

• Add listTasks to A2A JSON-RPC client
• Create fetchTasksForContext for server-side RSC task fetching
• Add convertTasksToUIMessages for task→UI message conversion
• Rewire AgentRun → PlatformContextProvider → MessagesProvider to use tasks
• Remove useListContextHistory hook and context history API code

Test plan

• mise run check passes (linting, type checking, doc snippets)
• Start adk-server + agent, verify conversation history loads in UI via A2A ListTasks
• Verify sidebar still lists conversations (contexts endpoint unchanged)
• Verify canvas edit works (artifact lookup via load_history)
• Verify new conversations work end-to-end