Allow AuthCheck to inspect invocation details for resource-level authorization

Enhancement

AuthCheck can decide whether a caller may use a tool, but it cannot authorize the specific resource referenced by a tool invocation.

For example:

from fastmcp import FastMCP
from fastmcp.server.auth import AuthContext

mcp = FastMCP("demo")

def authorize_invoice(ctx: AuthContext) -> bool:
    account_id = ctx.token.claims["account_id"]

    # Need the invoice_id from this invocation here
    # to check whether it belongs to account_id.
    return ???

@mcp.tool(auth=authorize_invoice)
def get_invoice(invoice_id: str) -> str:
    return invoice_id

AuthContext exposes the token and component, but not the arguments of the current invocation.

Expected: the auth check can authorize the requested invoice_id before the tool runs.

Actual: resource-level authorization has to be implemented inside the tool or separately in middleware.