Remove redundant None checks from Context methods

[jlowin]

The Context.fastmcp property already handles None checks internally, raising a RuntimeError if the FastMCP instance is no longer available. The property is typed to return FastMCP (not optional), so accessing ctx.fastmcp will either return a valid instance or raise an exception - it can never be None.

This makes the if self.fastmcp is None checks in these methods unreachable:

• list_resources()
• list_prompts()
• get_prompt()
• read_resource()

@property
def fastmcp(self) -> FastMCP:
    """Get the FastMCP instance."""
    fastmcp = self._fastmcp()
    if fastmcp is None:
        raise RuntimeError("FastMCP instance is no longer available")
    return fastmcp

This cleanup improves code clarity by removing defensive checks that can never trigger.

Fixes the outstanding request from PR #2249.