Handle SystemStackError in Execution::Next

[ydah]

This PR extends the execution-time SystemStackError handling added in #5683 to GraphQL::Execution::Next.

Previously, GraphQL::Execution::Next allowed a SystemStackError raised during execution to escape from GraphQL::Execution::Runner. This bypassed Schema#query_stack_error and returned an unhandled application error instead of GraphQL-Ruby's configured stack error response.

The runner now delegates execution-time stack errors to Schema#query_stack_error, matching the legacy interpreter behavior.

[rmosolgo]

I'm open to including this, but I'm curious, did you encounter it in normal use? Do you mind sharing a backtrace?

Execution::Next doesn't use recursive method calls like the old runtime did, so I'd be surprised if it raised these from its own execution.

If these come from application code, would rescue_from ... work to handle them?