Create set of tool names once in `getCoreSystemPrompt`

[jerop]

For performance, it's better to create the Set of tool names once at the beginning of the getCoreSystemPrompt method, rather than creating it inside this factory function every time it's called. This also allows you to reuse the Set for other checks.

I suggest adding this at the top of the function:

const enabledToolNames = new Set(toolNames);

Then you can use enabledToolNames.has(t) here. This would also be beneficial for the primaryWorkflows section, where toolNames.includes(...) can be replaced with enabledToolNames.has(...) for an O(1) lookup instead of O(n).

            planModeToolsList: PLAN_MODE_TOOLS.filter((t) =>
              enabledToolNames.has(t),

Originally posted by @gemini-code-assist[bot] in #18258 (comment)