[mcp-2026] Hide tasks on 2026 wire until tasks/update exists

[ROHIT-JR]

Closes #25

• Hide Tasks capabilities and task RPC handlers on auto/modern MCP 2026.
• Reject task-augmented tools/call with invalid parameters before background work starts.
• Preserve TaskManager and task behavior for legacy/in-process use.
• Remove the unused task notification module.
• Update tests so modern methods return method-not-found and legacy task coverage remains explicit.