Deprecate ctx.elicit() without response_type

[jlowin]

Calling ctx.elicit(message=...) without a response_type (or with response_type=None) generates a requestedSchema of {"type": "object", "properties": {}}. That's a valid JSON Schema but semantically meaningless under the current MCP spec's form-mode elicitation, and clients like VS Code render it as an empty form with a Submit button that does nothing — the tool call just hangs. See #3910.

This PR emits a FastMCPDeprecationWarning at call time pointing users at an explicit response_type. Behavior is unchanged for now so existing code keeps working; the None path will be removed in a future major. The warning respects fastmcp.settings.deprecation_warnings like the rest of FastMCP's deprecations.

@mcp.tool
async def confirm(ctx: Context) -> str:
    # Deprecated — triggers FastMCPDeprecationWarning, hangs in VS Code
    await ctx.elicit(message="Proceed?")

    # Preferred — real schema, renders correctly everywhere
    result = await ctx.elicit(message="Proceed?", response_type=bool)

Closes #3910