Hide the 'Run as task' switch for modern-era servers (task creation is server-directed) #2674

Problem

On a modern-era (2026-07-28) connection that has negotiated the io.modelcontextprotocol/tasks extension (SEP-2663), the Tools tab's Run as task switch is shown for every tool. It shouldn't be there at all on that era: under SEP-2663, task creation is server-directed. The server decides per request whether to return a task handle. The client has no "run this as a task" request to make, so the switch offers a choice the protocol doesn't have.

The current behavior is deliberate (clients/web/src/components/groups/ToolDetailPanel/ToolDetailPanel.tsx, the showRunAsTask / effectiveRunAsTask block and its NOTE comment). The switch "just declares intent to poll a returned handle". But that intent isn't something the user should have to supply. If the server can return a task for any call, the Inspector should always be ready to handle one.

Proposed change

• Hide the switch on modern era. Show it only for legacy (2025-11-25) servers that advertise tasks.requests.tools.call, gated on the tool's execution.taskSupport the way it is today (optional → user choice, required → forced on and disabled, forbidden → hidden).
• Route modern calls without the switch. Today useServerCommands.tsx's callTool picks callToolStream over callTool only when runAsTask is set (or the tool is required). With the switch gone, a modern connection with the tasks extension negotiated has to take the task-capable path for every call, because that path is what declares the extension on the request and makes an unsolicited task handle legal. Otherwise a server-returned task would go unhandled. Before choosing between "always callToolStream on modern" and "make callTool accept a returned handle", check what callTool does with an unsolicited handle today.
• Remove the modern-specific branches from ToolDetailPanel (modernTasks OR-ing into showRunAsTask / effectiveRunAsTask), and drop the modernTasks prop through ToolsScreen / InspectorView if nothing else needs it.
• Check whether the TUI and the CLIs have an equivalent per-call task option on modern, and apply the same rule there.

Acceptance

• Modern server with the tasks extension: no Run as task switch on any tool, and a tool call the server turns into a task still shows up on the Tasks screen with live status and progress, same as today with the switch on.
• Modern server without the tasks extension: no switch (same as today).
• Legacy server: behavior unchanged.
• Update the tests and stories for ToolDetailPanel / ToolsScreen that cover the modern switch.

Related: #1631 (modern-era tasks extension), #1071 / #1118 (earlier "Run as task" visibility issues).